Subject: Re: W6JWS-2m Intermod better guess(tm).
From: Jeff Liebermann <[email-masked]>
To: [email-masked], Cap Pennell <[email-masked]>, JV <[email-masked]>, Austin Lesea <[email-masked]>, 'Mike' <[email-masked]>
Date: Sat, 15 Jun 2013 08:46:39 -0700

Please ignore the previous bad IMD calculation and guess.

The IC-RP1510 receiver looks like this (everything in MHz).

 146.145       1st     21.8     2nd    0.455    demod and
   RF in ----> Mix ---> IF----> Mix ---> IF ---> RX audio
                |               |
                |               |
  5.12 ------> 124.300        21.345
  Ref Osc      1st LO         2nd LO

There is also the 146.745 transmit frequency, and 91.1 FM
broadcast station frequency involved.

This time, I think I found the intermod mechanism.  As usual,
nothing is simple until the answer is found.  Then, it looks easy.

It's called "Half IF" and it's about as obscure as it gets.
http://www.radioing.com/eengineer/rfi.html
What happens is that an incoming signal (as generated by an
intermod mix), mixes with 146.745 transmit signal, to produce
one half of the 1st IF frequency.  The half IF is then multiplied
by two in the fist mixer, to produce the 1st IF frequency,
and ruin my day.

The half-IF frequency is:
  146.145 - (21.8/2) = 146.145 - 10.9 = 135.245

This frequency is produced by the IMD mix as [1]:
  TX-freq - (3 * 1st-LO) + FM-station =
  146.745 - (3 * 124.3) + 90.910 = 135.245

The 1st LO then mixes with the spurious 135.245 signal
to produce:
  124.3 - 135.245 = 10.945

The 2nd harmonic of the above mix lands in the middle of
the 2nd IF as:
  10.945 * 2 = 21.89
Close enough considering the bandwidth of the FM broadcast
occupied bandwidth [1].

Unlike my previous guess(tm), this IMD mix has all the necessary
components.  It involves both the repeater TX frequency and
the FM broadcast station.  It does NOT involve any clock oscillators
which is unlikely due to the repeater shielding.  So far, it looks
right.

The 1:1 relationship with the transmit repeater TX frequency
explains why there was feedback squeal and why it was NOT
over-deviated feedback, as it would be if it were a multiple
of the TX frequency.

The fix is fairly easy.  Get the FM signal out of the repeater
receiver.  Exactly how to do that will require some sophisticated
testing (i.e. trial-and-error), as well as building a suitable
FM notch filter (i.e. cut-n-try).  A spectrum analyzer isn't
going to help much.  Since the mix involves the 1st LO frequency,
reinstalling the RF preamp that's sitting next to the repeater
might be possible.

[1] Note that the FM broadcast frequency that I used is not
91.1 rather 90.910.  That is within the approximate occupied
bandwidth of an FM station as calculated by Carson's Rule:
  Occupied Bandwidth  =
  2 * (deviation + max_audio_freq) = 2 * (75Khz + 53KHz)
  = 256Khz
The 53KHz is the highest modulation frequency without RDS
and SCA sub-carriers.
The range of occupied spectra would therefore be between:
  91.1 - 0.256 = 90.84 MHz
  92.1 + 0.256 = 91.35 MHz
http://www.axino-tech.co.nz/documents/Bandwidth%20needed%20for%20FM%20broadcasts.html

-- 
Jeff Liebermann                    [email-masked]
150 Felker St #D   http://www.LearnByDestroying.com
Santa Cruz CA 95060           http://802.11junk.com
Skype: JeffLiebermann         AE6KS    831-336-2558