EMRFD Message Archive 5506
Message Date From Subject 5506 2010-12-06 09:51:54 steve white HYCAS IF/AGC Advice I am building up the HYCAS IF/AGC amplifier board and it is designed for a
9Mhz IF. I am working on a receiver which has a 455 Khz IF and would like
to make the appropriate changes to the board in order to use it there.
Seems to me that I should just be able to scale the inductors and the input
circuit in order to make it work at 455. Does this sound right to
everyone??? Keeping the inductive reactance the same as at 9Mhz I come up
with values for L2 and L3 of 925uh. At 9 Mhz these are 47uh parts. For the
input I come up with 130uh and 600pf instead of 6.8uh and 30 pf.
Do my numbers pass the sanity test?
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5507 2010-12-06 11:38:51 Wes Re: HYCAS IF/AGC Advice Hi Steve, and group,
Yea, you are doing the changes that are needed. The 47 uH parts (L2, L3) are just RF chokes. The real load is determined by the resistors that parallel the chokes and T1. (R1, R2, R15) The gain actually peaks a bit lower in frequency than 9 MHz. For 455, you can just throw some wire
5509 2010-12-06 17:24:33 steve white Re: HYCAS IF/AGC Advice Wes
Thanks for the reply. Glad my guess was correct on what to do to move the
freq to 455.
I am building up the 9Mhz flavor right now and doing some trouble shooting.
I get considerable distortion when I turn the IF gain control to maximum
and I don't think that is right. I can easily get 100mv p-p without any
distortion at all. I have made the changes per the errata sheet. I have
reduced the value of R1 in order to reduce the gain of the second cascade
stage and have added the 22 ohm resistor in series with C12 for the emitter
feedback in the differential pair. I am thinking that I just have too much
gain and that I should change R2 to a 1K resistor and perhaps even add more
emitter feedback. This is the real question, to increase emitter feedback
do I just increase the value of the resistor that is in series with C12 from
22 ohms to say 30 ohms or do I increase the value of the capacitor??
5510 2010-12-06 18:36:42 Wes Re: HYCAS IF/AGC Advice Hi Steve,
An output level of 100 mV p-p, if it is across 50 Ohms, is -16 dBm. This is pretty loud and is close to the level that would overload a diode ring product detector. The nominal output for the Hycas IF strip is -30 dBm or there abouts. So, it is correct that the thing will distort with levels beyond this.
If you want to increase the emitter degeneration, increase the value of the resistor. Try something as high as 100 Ohms or perhaps even more.
Have fun with the thing.
5513 2010-12-07 20:39:25 steve white Re: HYCAS IF/AGC Advice Thanks Wes
I think I am in good shape now. I have the oscillation under control and
had to reduce the value of the gate resistors on Q4 and Q6. These were
originally 2.2k and I took them down to 1.0k. That did the trick. By the
way, I did not have a FB43-2401 for T1 so I used a FT37-43 and wound it with
the same turns ratio 16t:4t. This seemed to work. Not sure that this was
the right thing to do, does this sound acceptable?
Now to work on the 455Khz version.
5514 2010-12-08 11:06:52 Wes Re: HYCAS IF/AGC Advice Hi Steve,
Sure, the FT37-43 is virtually the same thing. I tend to use the beads instead of the "toroids" because they are cheaper. They are almost the same size, so use what you have in your stock of stuff.