Please look at circuit in question.
Side Band detector
1u || audio out +---------+-------+---+||+---+ | 8.4 + | || | --- | | --- 0.022| + | | 7.8Mhz RX sig .87 |/ GND | inject here +----+----------+| .-. | | |> | | .-. .-. 0.226 | | | | | | |7.8mhz carrier '-' 10k| | | |100k .-.inject here | '-' '-' | | 9.4V | | | | GND '-' 9.47 | +-----+ .-. | | | --- 680 | | ---1u '-' | +-----+ | === GNDTHe above circuit is also used in the detection section of the AM receiver and that works fine there because the 7.8Mhz carrier is not active at the emitter and there is no noise.
Problem, everything in this circuit seems to be fine with the manual etc.. The problem is, while in a SSB mode and very low 8.7mhs RX signal, the injected 8.7Mhz carrier at the emitter is generating a level of white noise, which can he heard in the audio section when advancing gain and reducing RF gain to nothing, it seems to be excessive. This noise would remind you of listening to FM with no signal, rush noise.
Using the Tek 485, I see the carrier (BFO if you want), at 2.1VPP with a .5us sweep. It matches exactly what should be there, according to the manual, however, It appears that this carrier is phase shifting some how? The 485 is displaying it in a widen fat line view, which tells me it is being Phase/Freq modulated some how with possibly some foreign noise ? I am on vacation so I don't have access to the lecroy I would normal use to measure this. I do have a Regal I could connect up but I have a hard time getting away from my Tek485 :)
Looking at the circuit above, the audio out, my scope at 50mv input with the 10:1 probe hardly shows any noise but it is there because the next audio stages amplify it nicely. I do have active fet probes with a preamp if that is needed. I also have other RF tools if I have to get out the big guns.
THis noise is reminding me of what you'd see at the base of a BJT in R.F. surroundings or just lots johnson noise, but I can't seem to find noise at the base, but a little barely showing at the emitter.
Could there be a problem with the 7.8mhz inject carrier at the emitter , producing this noise, due to some other issue else where? Like the
7.8mhz shifting being detected at this stage. Do you think I would be able to detect this as actual audible noise with this type of config?I have not yet sub the signal with the use of the HP gen, that's next.
As far as I know, the 485 should show a stable view of that at a .5us sweep. It has never failed me before.
I do have a other tools to work with here if needed. It's beginning to look like a modification to the circuit may be needed.
Jamie