DaveInDenver
Middle Income Semi-Redneck
What you're saying is true in general.
The FT-857 does derive its AM TX signal from the same synthesizer and 1st IF mixer (an SA602 in the FT-857), which they unbalance when in AM mode and leave the synthesized carrier (they use an AD9835). When in SSB mode they use filters to strip the carrier and select the sideband after the AF is mixed with the DDS output. This is a common method in multimode radios that's been used for years even before DDS. There's just no way Yaesu or anyone else is going to put in a dedicated AM modulation path when there's already a perfectly useful one.
There are separate IF filters for AM and SSB/CW by necessity, but other than this and the soft settings in the AF DSP, the IF and the PA paths are shared. They do, as you mention, handle FM differently in the 1st IF where they have a fixed spatter filter (limiter) and deviation circuit as a result. This is a design compromise otherwise they have a significant increase in components and circuits to do all modes on all bands and being able to do SSB on VHF and UHF or FM on 10m and 6m is part of the flexibility. High end radios with the space and IF DSP available don't share quite as much and implement better modulation limiters. The main criticism of the FT-857 is that it's a 3KHz bandwidth at most, so even using a better IF filter wouldn't make much of a difference, although it would make mic gain less critical.
You are correct that a CB PA when it runs out of head room creates the more graceful flat top on over modulation as a true full carrier modulator. No argument there. But the downside is you exceed your TX bandwidth as a side effect and generate significant harmonics at the output, something that's relatively difficult to do in the case of the FT-857 with just mic gain. After the 1st IF you generate a heavily distorted signal which would sound like a carrier with garbled information. Even when set correctly the criticism is that the DDS is 10-bit, so it's never going to sound super high fidelity when monitored on a fully analog AM receiver. That is ultimately the CD-vs-LP argument (and I do agree there, vinyl sounds better). It's just not practical nor I would argue necessary for communications to bother doing all that, even for AM. If you want to sound like a commercial AM broadcaster then a communications grade transceiver is not your best choice.
For the best AM in this class of radio, turn off compression and set the AM mic gain to prevent IF distortion and be sure the SWR remains 3:1 or lower. With the output set to 5W on AM the ALC will only roll back on reflected power since the PA doesn't have much to do, it would be likely doing something like 3dB to maybe 6dB set to min power and can do about 26dB absolute gain. Flat topping the output is a problem when you run head room of 6dB or less on AM, which is about where you'd be running set to 25W on a FT-857. Set to 5W the PA is barely doing anything on the carrier, so the full power is available for peaks so even if you scream into the mic there's no way the ALC is going to roll back for exceeding the envelope. The IF doesn't have the head room, so as you've identified distortion there could be a real issue, definitely. Again, mic gain is critical.
My statement about the Uniden is more unspecific, I would have no issue aligning my CB if it was legal to do so (I have tools to build and test radios with the exception of a dedicated FM service monitor, although I can hobble one together with stand-alone instruments). But even so since it requires internal set pots to adjust even monitoring the output isn't going to help much beyond letting you know if your mic element is too weak or strong and how far to keep it from your mouth when speaking. That's all I was saying, that the radio is pretty simple and lacks even basic external function to adjust mic gain or compression.
The FT-857 does derive its AM TX signal from the same synthesizer and 1st IF mixer (an SA602 in the FT-857), which they unbalance when in AM mode and leave the synthesized carrier (they use an AD9835). When in SSB mode they use filters to strip the carrier and select the sideband after the AF is mixed with the DDS output. This is a common method in multimode radios that's been used for years even before DDS. There's just no way Yaesu or anyone else is going to put in a dedicated AM modulation path when there's already a perfectly useful one.
There are separate IF filters for AM and SSB/CW by necessity, but other than this and the soft settings in the AF DSP, the IF and the PA paths are shared. They do, as you mention, handle FM differently in the 1st IF where they have a fixed spatter filter (limiter) and deviation circuit as a result. This is a design compromise otherwise they have a significant increase in components and circuits to do all modes on all bands and being able to do SSB on VHF and UHF or FM on 10m and 6m is part of the flexibility. High end radios with the space and IF DSP available don't share quite as much and implement better modulation limiters. The main criticism of the FT-857 is that it's a 3KHz bandwidth at most, so even using a better IF filter wouldn't make much of a difference, although it would make mic gain less critical.
You are correct that a CB PA when it runs out of head room creates the more graceful flat top on over modulation as a true full carrier modulator. No argument there. But the downside is you exceed your TX bandwidth as a side effect and generate significant harmonics at the output, something that's relatively difficult to do in the case of the FT-857 with just mic gain. After the 1st IF you generate a heavily distorted signal which would sound like a carrier with garbled information. Even when set correctly the criticism is that the DDS is 10-bit, so it's never going to sound super high fidelity when monitored on a fully analog AM receiver. That is ultimately the CD-vs-LP argument (and I do agree there, vinyl sounds better). It's just not practical nor I would argue necessary for communications to bother doing all that, even for AM. If you want to sound like a commercial AM broadcaster then a communications grade transceiver is not your best choice.
For the best AM in this class of radio, turn off compression and set the AM mic gain to prevent IF distortion and be sure the SWR remains 3:1 or lower. With the output set to 5W on AM the ALC will only roll back on reflected power since the PA doesn't have much to do, it would be likely doing something like 3dB to maybe 6dB set to min power and can do about 26dB absolute gain. Flat topping the output is a problem when you run head room of 6dB or less on AM, which is about where you'd be running set to 25W on a FT-857. Set to 5W the PA is barely doing anything on the carrier, so the full power is available for peaks so even if you scream into the mic there's no way the ALC is going to roll back for exceeding the envelope. The IF doesn't have the head room, so as you've identified distortion there could be a real issue, definitely. Again, mic gain is critical.
My statement about the Uniden is more unspecific, I would have no issue aligning my CB if it was legal to do so (I have tools to build and test radios with the exception of a dedicated FM service monitor, although I can hobble one together with stand-alone instruments). But even so since it requires internal set pots to adjust even monitoring the output isn't going to help much beyond letting you know if your mic element is too weak or strong and how far to keep it from your mouth when speaking. That's all I was saying, that the radio is pretty simple and lacks even basic external function to adjust mic gain or compression.
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). Having a mic gain control does allow you to adjust it for reduced background noise in a noisy vehicle (and most of your better CB units do provide one), however it's not something absolutely essential like it would be on a HF amateur radio.