My own testing and measurements:
Out-of-the box (and even after the so-called "MARS-CAP" mods) the receive frequency range (for the standard USA version) is as follows:
Cramming all of this capability into such a small box is really quite an amazing feat. One should not, however, expect that such a small, relatively inexpensive radio will have receiver performance equal to the highest-end receivers out there. When I read about people complaining of some relatively minor performance deficiency (such as noise blanker operation or the presence of birdies) then I have to refrain from dashing off an email reminding them that the FT-817 is not and was never intended to be the penultimate receiver. Nevertheless, its performance is really very good in the amateur bands.
Soon after purchase I utilized several pieces of test equipment available to me (i.e. service monitors, etc.) One of the first things that I did was to check the receiver sensitivity and image rejection on various frequencies - some aspects of which have not (to my knowledge) yet been reported elsewhere.
Note: These measurements were performed on a
FT-817 using the original configuration parameters.
A few notes about these measurements:
Along with the radio's schematic, this chart can tell us a few interesting things:
"What is Image Response?"
Every superheterodyne conversion scheme has what is called an image frequency. Taking the FT-817 as an example, the first IF is at 68.33 MHz (except for the FM broadcast band, but more on this later.) Whenever the 68.33 MHz IF is used, the local oscillator is always 68.33 MHz above the receive frequency: When receiving at 10 MHz, the local oscillator is at 78.33 MHz, and for 440 MHz, the local oscillator is at 508.33 MHz.
There's also another false receiver response called an image. If a filter were not used, our receiver (in the above example) would receive a signal at a frequency of twice the IF frequency (i.e. 136.66) MHz above the receive frequency. Taking the example in the previous paragraph, this would mean that in addition to our 10 MHz signal, we'd receive something at 146.66 MHz, and in addition to our 440 MHz signal, we'd receive something at 576.66 MHz.
As you can see, having a fairly high first IF frequency - 68.33 MHz in our case - means that we need to filter out a signal that is 136.66 MHz away from where we want to listen. If we had used an IF frequency of, say, 1 MHz, the image would be only 2 MHz away. At AM broadcast band frequencies this sort of filtering isn't too hard to manage, but at 2 meters or 440 MHz, trying to filter a signal that is only 2 MHz away requires complicated and/or expensive filtering: Filtering something that is over 136 MHz away is easy - even at 440 MHz.
Even with filtering, that image still does exist - but is greatly reduced. For clarity, here is a bit of explanation for the image response data on the chart:
On the FM broadcast band, the FT-817 doesn't use any of its high-dynamic range receive sections, but rather an all-in-one FM receiver chip - Q1025. Furthermore, it uses an IF of only 10.7 MHz, so its image is only 21.4 MHz away. Finally, its input filtering is rather minimal, using only the 2 meter diplexer and a single tracking filter. In other words, there was little incentive to put more money than absolutely necessary toward FM broadcast band reception.
From 76 to below 88 MHz, the local oscillator (LO) operates on a frequency 10.7 MHz below that of the receive frequency, but from 88 through 108 MHz, the local oscillator operates 10.7 Mhz above the receive frequency: This places the image 21.4 MHz below and above the receive frequencies, respectively.
Why the strange split? Running the LO below the receive frequency below 88 MHz places the image in the 54-66 range - placing its image well into the diplexer's highpass filter cutoff frequency. Running much higher than this with a low-side LO would reduce the filtering efficiency - right in the middle of the TV band, possibly resulting in hearing the buzz of TV carriers in that range. Above 88 MHz, a high-side LO is used, placing the image in the range 109-129 MHz. Even though the image response at 88 MHz and above is pretty terrible, all that is likely to be heard would be the occasional aircraft or airport transmission, and this would probably be much weaker than a high-powered FM broadcast station.
There is another problem with the FM receiver on the FT-817: Using the "single chip receiver" has the advantage of low cost and simplicity. Unfortunately, its dynamic range is pretty poor. What this means is that the FM broadcast band receiver is very prone to strong signal overload - something that can't help but be noticed in strong signal urban areas - especially when using a large antenna. It would have been nice if Yaesu could have routed that signal through a switchable attenuator for use in this frequency range, but...
Yes, you can hear NOAA weather on your FT-817. Simply tune in a frequency 136.66 MHz (2x the IF frequency) below your local (strongest) local NOAA frequency. That is, 162.55 MHz would be tuned in at 25.890 MHz using FM.
The problem? The filters in the '817 were designed to prevent this - because this is the image response. The NOAA broadcast will be a mere 85 db weaker than it would be were the receiver designed to work at this frequency. This means that your local NOAA broadcast must be really strong (i.e. you must be within a couple miles of the transmitter) for you to hear it this way - this is probably not going to work when you are backpacking...
Now, if you want to work around this, you could do one of several things (and no, adding an 85 db gain preamp isn't one of them...):
IF rejection in the FT-817:
You may have already figured it out, but a superheterodyne is actually a single-frequency (the IF frequency) receiver with a converter in front of it to translate the receive frequency to the IF frequency. It follows that it takes some care to keep signals on that IF frequency from getting into the receiver. In the case of the FT-817, that means that a strong enough signal at 68.33 MHz will "break" through and you will hear it no matter where you tune the receiver. The FT-817, therefore, has some filtering to keep that from happening. The performance of this filtering is as follows:
The FT-817 actually has two IFs in it: The 68.33 MHz IF is at a high frequency to simplify image filtering (as described above) but it is rather difficult and expensive to make a highly selective filter that operates at such a high frequency, so the filter that does operate there is fairly broad and only has mediocre selectivity. To simplify filtering, the 68.33 MHz IF is converted down to 455 MHz (where we can get some good filtering at much lower cost) by mixing it with a 67.875 MHz signal - a frequency which happens to be exactly three times that of the master reference oscillator - a fact that is not a coincidence.
This IF has an image, too: The 68.33 MHz IF filter is pretty narrow (10-20 KHz or so) but an image can occur that is 910 KHz (twice the 455 KHz IF) above the receive any frequency (except on the FM broadcast band.) This means that if you are tuned to, say, 10 MHz, it is possible to hear a signal that is at 10.910 MHz.
But how well filtered is this? As it turns out, the 68.33 MHz IF filter is able to attenuate this 2nd IF image such that it is 55db down. This means that the signal 910 KHz above the current receive signal must be over 300,000 times as strong as the one you are tuned to in order to show up on the S meter with the same strength. In actual operation, it is very unlikely that this image will cause you any problem, unless you have a next door neighbor that is operating exactly 910 KHz above where you are receiving.
This might then bring up the question: "If some of the 68.33 Mhz 1st IF signal can get through, then can a 455 KHz signal get into the IF from the antenna?" The answer is yes, it can. This was noted only in the AM broadcast band and in the 1.8 range. In the 1.8 MHz range, this "leakage" was down 95 db down. The "leakage" was a bit greater in the AM broadcast band, but it was still very minimal. Anyway, you'd need to have a very strong signal on 455 KHz in order for this to be any sort of problem - and that's not likely to happen - because there really aren't any!.
The Radio's Front End Filtering:
Front End filtering is one of several determining factors of receiver performance. As is typical of most modern HF amateur transceivers, the receive bandpass is set by cascading the transmit lowpass filter with a highpass filter: The highpass filter is switched out during transmit.
The coverage bands of the highpass/lowpass filters are set as follows:
The simple answer is: Because the FT-817 was designed that way.
It's fair to say that the FT-817 was designed to work with a 50 ohm antenna (i.e. a real antenna) on all frequencies.
The VX-5R, on the other hand, was designed to be able to use its rubber duck antenna for AM/SW reception. In looking at the VX-5R schematic, I noticed that not only is there a lot of extra gain in the signal path, but the receiver's input impedance is quite high - much higher than 50 ohms. This allows a small whip antenna to work better for operating on the AM/SW frequencies.
If you want to convince yourself that the FT-817 actually has a better receiver than the VX-5R, connect each one to a full-sized HF antenna: I connected a borrowed VX-5R to a triband beam and I had to put 30db (1000-fold!) of attenuation in front of the VX-5R to eliminate front end overload - an expected result of so much gain and the use of a "weak" mixer. Also problematic with using a large antenna in this frequency range is that there exists a pair of back-to-back diodes (D3004) at the RF input of the AM/SW/50 MHz receive section (used to protect the receiver during transmit) that could be driven into conduction, causing intermod.
In contrast, the FT-817 will happily work in such an environment with no attenuation at all.
A note about 222 MHz:
It is a shame that Yaesu did not see fit to put 222 MHz coverage in this radio. It would have required, probably, another $25-$35 of components (i.e. high/lowpass filters, maybe another VCO section, and another RF section for receive and transmit.) It should also be noted that the broadband nature of the power amplifier makes it perfectly capable of operation at 222 MHz.
Not including the 222 is not all that surprising,
It would be legal to use only in the U.S.A. and
few people would find it useful. It is, however, a
problem: The band isn't used much because equipment isn't readily
available. The equipment isn't readily available because
few people use it. The band isn't used much because equipment
readily available. The equipment isn't readily available
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This page maintained by Clint Turner, KA7OEI and was last updated on 20150415. (Copyright 2001-2015 by Clint Turner)