Using a surplus 1-watt TBQ-3018 VSAT amplifier module for 10 GHz amateur band operation

Typical installation of
                the outdoor unit
Examples of
                two of the outdoor units from which the power modules
                were obtained.
Figure 1:
Top:  Typical installation of an outdoor unit.  A feedhorn is attached and the unit is mounted in a carrier that permits rotational adjustment of polarity.  (Yes, those are 10-meter dishes in the background...)
Bottom:  Examples of two of the outdoor units from which the amplifier modules were obtained.  The feedhorn and "OMT" (Ortho-Mode Transducer) have been removed on these units as they may have been re-used.  The upper unit had been deployed near the ocean and was mechanically damaged:  Its amplifier module was the source of the one in the pictures below.
Click on a picture for a larger version.

Using the very amplifier shown on this page, the K7RJ 10 GHz beacon was put online on 11 September, 2011 from grid square DN31it operating on a frequency of 10368.250 +/- 500Hz.  Located approximately 110 miles northwest of Salt Lake City, it has been heard by a number of stations in the Salt Lake area and has been shown to be a reliable "weak signal" source.

This beacon uses Frequency-Shift CW (FSCW) with the beacon message consisting of an ID and grid square locator followed by a 30-second carrier at the "key-down" frequency.  This beacon operates at a power level of 1.0 watts feeding a horizontally-polarized omnidirectional slot antenna and the frequency stability is such that it is possible to use "waterfall" display programs (such as Spectran or Spectrum Lab) to detect it at sub-audible levels.

This page is a work in progress and it will be updated as time permits.



Working on the Satcom industry, I noticed a growing pile dead of Hughes "Tigris" ODU (OutDoor Unit) modules in the bone yard.  Made from the late 1990's into the early 2000's, these were various returns from the field and were, for various reasons, declared dead or not economically unrepairable.  Since it will often cost about as much to repair a unit as to replace it, "dead" units are often put in bone yard and used as a source for mechanical parts to repair other units.

The typical failure modes of these units may be grouped into two general categories:

Over the years it was noted that it's fairly rare that failure the component of interest in this case - the output power module - is actually the reason that the unit was pulled from service.  Being buried deep inside the unit, it seems to be fairly well-protected against electrical impulses and it is also unlikely that even some moisture ingress and related corrosion would be severe enough that the amplifier module and the circuitry surrounding it would be damaged to the point of being unusable.

As can be seen from the top picture in Figure 1, these units are typically mounted in a bracket that allows the polarity to be rotated to match that of the satellite at the earth station's location.  A circular feedhorn is typically used to illuminate an offset-fed antenna that is typically around 1 meter in diameter and this is coupled to an "OMT" (Ortho-Mode Transducer) that provides the diplexing between receive and transmit as well as separating signals from the two polarities.

The bottom picture of Figure 1 shows two such units having been removed from their mounting brackets along with the removal of the OMT.  The upper unit - having been deployed near an ocean coast - shows severe corrosion and some physical damage:  As the aluminum oxidized, it expanded and popped the waveguide flange apart and further damage was done when the screws holding waveguide feedhorn had to be chiseled off!  The lower unit fared much better, not having been exposed to such a corrosive environment and was likely pulled from service to do an electrical problem.

It is quite likely that a unit in condition similar to the upper (and more badly-corroded) will will yield usable parts for our application as it was probably not damaged by an impulse or other electrical mishap, but failed due to moisture ingress and/or mechanical failure - and the the board inside was likely (mostly) in good shape as the unit's ultimate failure resulted in immediate replacement.  It turned out that both units contained good amplifier modules and the upper unit showed only minor evidence of moisture on the board.


A brief description of the circuit:

In a nutshell, here is how these ODU modules work:

For our use we'll cut away a section of the circuit board and the portion that we need to extract contains two important pieces:
To complete the parts count we need only slice the trace on the input lead of the input MMIC and install a DC blocking capacitor:  This capacitor may be found on the remainder ("discarded portion") of the circuit board having been used to bock the DC for this very MMIC!
Preparing the amplifier module for use:

For the units that I've investigated the output power module is a Teledyne TBQ-3018 and it is for these units that this description is targeted.

Th TBQ-3018 is a 1-watt (nominal) GaAsFET MMIC module that is intended for operation in the Ku uplink frequency band of 14.0-14.5 GHz.  In digging around on the web, I was able to find a data sheet for this module and careful inspection revealed something interesting:  Although most of the specifications showed its use in the 13-15 GHz range, one chart in particular showed the 1dB compression output over a range of 11 to 18 GHz, and the power output at 11 GHz was still a respectable +27.5dBm, or about 560 milliwatts!


Having several of these units to mess with, I decided to try my hand at getting one to work in the 10 GHz amateur band, so here is a general description of the procedure:
                circuit board showing the cut-off portion with the power
The shim
                from underneath the power module. This shim should be
                saved as it is useful for both thermal and grounding.
Figure 2:
Top:  The entire circuit board from the outdoor unit.  The section containing the TBQ-3018 power amplifier is in the lower-left corner of this picture, having already been cut from the main board.
Bottom:  The small foil shim that was under the power module.  This piece should be carefully saved as it is used both for thermal conductivity and RF grounding.  The "tab" on the side opposite the mounting holes goes under the portion of the board with the output RF connector.
Click on a picture for a larger version.

                cast aluminum shield covers. The lower cover is
                unmodified, but the upper casting has the portion that
                covers the amplifier section removed.
The modified
                amplifier section of the circuit board.
Annotated picture showing the connection points of
                the amplifier
Figure 3:
Top:  The lower casting is the unmodified RF shield cover while the upper shows which portion was removed.  Be sure to save any pieces of RF absorbing material in the shield castings..
Center:  A modified, extracted amplifier board section with the "input" being at the bottom-left.  Extra holes have been drilled to accommodate the 3-hole SMA connectors.  Also note the cut in the trace on the left side in preparation for a DC blocking capacitor to be added for the input predriver MMIC.  The above picture shows an early test unit on which I'd broken some of the leads of the TBQ-3018 power module and resoldered - a reminder that the leads are VERY fragile!
Bottom:  Annotated picture showing the various connection points.
Click on a picture for a larger version.


Before we apply power, there are a few precautions to be taken:

(These points are important enough that I thought that I'd say them more than once...)

The modified VSAT power
                amplifier on the workbench
                close-up view of the amplifier module with the shield in
View of the
                amplifier showing the connectors.
Figure 3:
Top:  The repackaged TBQ-3018 VSAT amplifier under test on the workbench.
Center:  The amplifier installed in the die-cast box with the shield covers in place.  Notice the wires for the drain and gate bias emerging from holes in the shield casting.
Bottom:  The "other" side of the amplifier, showing the "3-hole" SMA connectors and the screws holding the board, connectors and shield covers in place.  Since these pictures were taken, feedthrough capacitors have been added and holes drilled and tapped to allow mounting of the amplifier to a heat-spreading plate.
Click on a picture for a larger version.
The recommended procedure is this:
You may now run RF tests!

When powering down, remember to remove the RF drive and drain voltage before you remove the bias voltage!

Observations and comments:

Using the amplifier - the K7RJ 10 GHz beacon:

One ongoing project is nearing completion - the K7RJ 10 GHz beacon.  This beacon will ultimately be placed at some property that Ron, K7RJ, owns that is located in the (remote!) northwest corner of Utah (grid square DN31) at a distance of 120-ish miles from Salt Lake.  Because it is line-of-sight to the mountains near Salt Lake City it should provide a weak signal source that should also demonstrate various weather-related propagation effects.  (It should be noted that contacts between Salt Lake and this location have already been made on 10 GHz.)

This beacon is fairly simple:  An OCXO (Oven-Controlled Crystal Oscillator) is located - along with the FSK-CW keying circuit - indoors, with a length of RG-6 coax to convey the 96-ish MHz signal and multi-conductor cable for both power and monitoring to a weathertight box located outdoors, at the antenna - an omnidirectional waveguide slot.  In this box is a "Brick" oscillator locked to the OCXO's output is then fed to the amplifier shown in Figure 3 which is set for an output power level of +30dBm (1 watt) and a frequency of approximately 10368.249 MHz.

It was for this project that I constructed two circuits:  An 8.2 volt switching regulator, and a bias control and current-limiting circuit.

8.2 Volt switching regulator:

Because the amplifier is located remotely, a 24 volt AC supply is used to minimize I/R drop and since an 8.2 volt DC supply at just under an amp was needed, a simple switching voltage converter was constructed to minimize both power consumption and heat generation within the confines of this box.  This circuit is based on the National Semiconductor LM2575-5 step-down converter which requires only a few extra components and because of its efficiency (in the 85-90% area) it produces relatively little heat and requires no heat-sinking other than that afforded by its being attached to a mounting plate inside the box.  The circuit depicted in Figure 4 requires that only low-ESR electrolytic capacitors are used throughout and it also includes additional L/C filtering on both the input and output lines to quash conducted 52 kHz energy from the switching supply to prevent its modulating the 10 GHz signal as well as being conducted outside the box and interfering with HF operations that one might conduct nearby!

The only "critical" component - aside from the electrolytic capacitors - are the inductors:  L302 should be rated for several amps and can be anything from 100uH to 470uH, but something in the general area of the 330uH specified is recommended.  Because they are used for filtering, the precise values of L301 and L303 are unimportant and I simply used some toroidal chokes plucked from some scrapped switching power supplies:  Anything from 47uH and up may be used as long as it can handle at least an amp with minimal voltage drop - particularly L303.

YouTube video showing the K7RJ 10 GHz beacon using the amplifier described on this page.

The LM2575-5 regulator was used rather than the "LM2575-ADJ" since the former was on hand and the latter was not, but the "ADJ" version could have been used if the values of R301 and R302 were appropriately selected for the desired output voltage or, in either case, if R301/R302 were replaced with a potentiometer in the 1k to 5k range connected between the output at C303/L302 and the wiper connected to the "FB" pin.  Because of the slight voltage drop in the amplifier bias control and current limit circuit described below and interconnecting wiring, the voltage from this regulator was set slightly high - to 8.2 volts - so that under load, close to 8.0 volts was obtained at the amplifier itself.

In the K7RJ beacon, the main power supply was from a 24 volt AC transformer which was half-wave rectified twice:  Once to provide the negative voltage for the "brick" oscillator, and again to provide the positive supply for this switching regulator and the controller board, below.  The 8 volt switching regulator an the 7812 regulator on the controller board (plus another switching regulator on the negative supply bus used to power the "brick) are capable of operating from the 28-32 volt DC bus, this 8 volt switching regulator is perfectly capable of running from as low as 11 volts.

When initially testing the beacon as a whole it was noted that heat buildup in the outdoor box was the main technical problem to be solved!  Enclosed in the weatherproof, fiberglass enclosure, a temperature rise of over 70 degrees F could be expected even with the use of the switching regulator, and if a linear regulator had been used, the heat problem would have been much worse!  To keep the electronics inside at a reasonable temperature, passive thermal transfer technique were employed to maintain safe levels - especially on sunny, 100+ F days!  Another tactic was to construct a sun shield for the box using a small solar panel which is then used to power a fan that moves air through the box.

Amplifier bias control and current limiting circuit:

The TBQ-3018 amplifier chip requires a bit of external bias control and monitoring circuit to keep it "happy" - namely, to maintain the drain current at a safe and consistent level under varying conditions.  To do this, the circuit needs to monitor the drain current and dynamically adjust the bias voltage to maintain that current.  An additional feature is a current limiter that offers a bit of extra protection to the amplifier in the event that the bias line be accidentally disconnected or shorted out during testing.  Two supply voltage are required:  An 8.2 volt supply for the amplifier module itself, and a higher voltage for the op-amp and voltage converter - nominally 12 volts.  In this particular case - since the main V+ supply is in the 28-32 volt area, a 12 volt 3-terminal regulator (a 7812) is used to provide the lower voltage while a 5 volt regulator serves as an onboard voltage reference.  If a regulated 12-15 volt supply were available, the 7812 regulator could be omitted entirely and the op amp and 555 negative voltage charge pump could operated directly from that.

Schematic of
                the 8 volt switching regulator
Schematic of
                the TBQ-3018 amplifier bias controller/current limiter
Figure 4:
Top:  The schematic of the 8 volt switching regulator.
Bottom:  The schematic of the bias control and current limiter.
Click on a picture for a larger version.
Circuit description - see Figure 4:

U401A, Q401 and R401-R403 form a high-side current sense circuit with the voltage on the emitter of Q401 representing 5 volts for one amp of current flowing through R401.  The output from the emitter of Q401 is applied to U401D which is compared with the voltage from the wiper of R406 and if it exceeds the threshold, the output of U401D goes up and the P-Channel MOSFET Q402 is turned off, limiting the current to the amplifier to the pre-set level.  D408 is necessary as without it, this circuit can "latch up" if the output the output goes to "full scale" (e.g. the output is short-circuited) - a condition caused by the base-collector junction of Q401 conducting back to U401A's non-inverting input if its output goes above about 8 volts.  An LM324 is used for this circuit because it is capable of operating down to the negative rail on both the input and output:  If another type of op-amp is used, make certain that it has this capability!

For bias regulation, output voltage from the emitter of Q401 is first buffered by U401B and then applied to U401C which compares the voltage at the wiper of R409 with the voltage from the current sense circuit:  If the current is too low, the output of U401C goes up and pulls the "Amp Gate Bias" closer to zero (e.g. less negative) which increases the drain current, but if the drain current is too high, the output from U401C drops which permits the bias line to become more negative to decrease drain current and in this way, the drain current is dynamically regulated.  Zener D401 limits the positive swing from U401C to 5.6 volts, which, after passing through the voltage divider consisting of R412 and R413 (plus a 350 ohm resistor on the amplifier board itself) that can vary from about 0 volts (e.g. 5.6 volts at the top of D401) to about -1.2 volts when U401C's output is at zero volts.

Because the amplifier needs a negative bias voltage, U402, a 555 timer, and associated components are used to form a charge-pump voltage converter using D404, a red LED, as both a power-on indicator and as a regulator to clamp the voltage to the 1.9-2.1 volt region.  As noted above, a voltage divider consisting of R412 and R413 (plus the aforementioned resistor on board the amplifier itself between the bias line and ground) serves to provide an adjustable negative bias that will "servo" to maintain a constant drain current under all drive and temperature conditions.

Diode D104 and transistors Q402 and Q403 are used to detect a failure of the negative supply  which would cause the bias voltage to go positive and saturate the amplifier, putting it into current limiting.  If the voltage from the bias generator disappears, Q402 turns off which, in turns, turns on Q403 which applies a positive voltage to the gate of Q402, the P-Channel MOSFET (which should be attached to a small heat sink) and turns it off, disconnecting the amplifier's drain supply.


There is one potentially dangerous condition that is not detected by this circuit:  The loss of the 12 volt supply if the 8 volt supply is still present.  If this occurs, the amplifier may be damaged as Q402, the P-Channel MOSFET, will be fully turned-on, no (negative) gate bias will be present - turning the amplifier module completely "on" - and no current limiting will occur!  Admittedly, this particular condition was not anticipated when the circuit was first designed, but adding a simple PNP transistor/resistor/diode circuit around Q402 could easily be used to detect if the "+12" volt supply bus drops below 8 volts, turning on the PNP and forcing Q402 off.  This is not shown in the schematic, but I may add it later.  It is worth mentioning that the aforementioned condition did accidentally occur during testing in the beacon and the 8 volt switching regulator current-limited at some unknown value (probably over 2 amps) but the amplifier was apparently unharmed!

Adjustment and setup:

The current limiting should be set to be about 20% higher than the desired drain current and this can be approximately calculated by measuring the voltage across the wiper of R406 and dividing by 5.8 to yield the current in amps.  This may also be verified simply by using an ammeter between the source of Q402 and ground and measuring the short-circuit current.  An alternative method is to simply set the wiper of R406 to the high side (+5 volts), verify that the current-limiting works, and then set the bias point for operation.  One would then decrease the setting of R406 until current limiting just occurs, measure the voltage on its wiper, and then adjust it upwards by 20%.

ONLY after it has been verified that the current limiting circuit is operational, the amplifier may be connected after first pre-setting the wiper of R409 to ground to set the current to minimum.  While watching the current drawn by the amplifier - which may be done with either an inline ammeter or by measuring the voltage on pin 7 of U401 and dividing by 5 - slowly increase the current using R409 until it reaches the desired level.  If the drain voltage suddenly drops with increased current, it may be necessary to (at least temporarily) increase the setting of R406 in the event that the current limiting is activating.

As with many FET amplifiers, the gain is related to the bias and the saturated output power level will increase with higher current.  Clearly, there is a limit of safe power dissipation for this (or any) device - partially based on how well heat-sinked the device is and also its maximum ratings.  The data sheet that I found for the TBQ-3018 is a bit unclear as to what the absolute maximum operating drain current of the amplifier is, but 850 milliamps and 1 amp are figures noted in the data sheet and testing indicates that this amplifier can operate at 850 mA for long periods and at 1 amp for at least a short while if well heat-sinked.

It was noted on the amplifier shown in Figure 3 that with about +2dBm of drive, a drain current of about 750 milliamps yielded a saturated power output of about +30dBm (1 watt) at 10.368 GHz which was the desired output power for the beacon. 

Still to do:

Update:  This amplifier is currently undergoing testing for long-term use in the K7RJ 10 GHz propagation beacon at an output power level of 1 watt.  Thus far, the results look promising!
Update:  I've built and tested a regulating power supply for this amplifier in preparation for its use in the K7RJ 10 GHz propagation beacon.  This supply uses a switching regulator to provide the 8 volt drain supply, it has both overcurrent and bias-fail protection and it uses servo techniques to dynamically set the bias voltage - see above!
Update:  When this amplifier was installed for use on the K7RJ 10 GHz propagation beacon, its die-cast box was bolted to an aluminum plate to provide additional heat-sinking.  Testing has shown that this degree of heat dissipation is entirely adequate.

Additional notes:

Go to the KA7OEI microwave page, or go to the KA7OEI main page.

If you have additional questions, you may send email using the link at this page.

This page updated on 20160720

Since 5/2011: