RadioBanter - Icom R-75 question

On Sun, 25 Jan 2004 01:50:53 -0500, "phil :)"
wrote:

hi Telamon:

A more complex reason for the RX340 non-lock is that the radio has a
front-end analog RF AGC followed by a DSP AGC. The locking problem could
be related to the way the two loops interact in various reception
situations.

the RX340's 16-bit ADC is not ideal as this limits AGC inside the DSP. the
"software" AGC must keep altering [via DAC] the analog gain during fades.
new ham gear uses 24-bit ADCs.

regards,
phil :)

Right on, Phil.

2nd generation (16-bit) DSP IF receivers (RX331/340, WJ8711A/HF1000)
with this architecture have this problem. (The first generation
Collins HF2050 only had 7-bit A/D, but also a different architecture.)

I find that by using the IF gain control to place the signal amplitude
in the sweet spot where its variations stay within the 16-bit range of
the 3rd IF I very seldom have unlock/relock problems.

However, if I get fading so great that it requires the outer loop to
adjust that IF gain to keep the signal in the DSP range, if the fade's
rate-of-change is sufficiently high the old unlock/relocks reappear.

If you need 8-bit "telephone quality" dynamic range for the
modulation, the carrier can only have 8 bits. Even with the more
reasonable assumption of 6 bits for program content, you still only
have 10 bits - or 60dB.

On the other hand, with 24 bits you'd have 96dB even with 8-bit audio
(to maintain full fidelity of all the static and crud on MF/HFg).

Even with this limitation, however, my RX340s totally outclass my
Harris, Cubic and Icom receivers.

Cost-reducing $10k (or more) surveillance receivers to $4k is
difficult enough, and I have a lot of respect for Ten-Tec's engineers.
The addition of SAM was designed for the "human-friendly" RX340 and
then back-fitted into the RX331, since it was IF-software-only.

Now if the spooks would just order a next-generation to the 331/340
with 24-bit A/D.......g

This is where someone like Winradio who uses your PC's soundcard for
the last IF could perhaps have a software/hardware verison that
exploited 24-bit soundcards. Unfortunately, unless I'm mistaken, their
software runs the same on any "compatible" (with SB16) soundcard.

Probably a wise business decision due to the much more limited market
of folks with 24 bit soundcards. Oh well, they'd probably have to
spend more on the front end stuff, anyway.

Good listening,
Al
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Location: 42N39, 71W09 (Near Boston, MA)
HF Antennas: 65ft TFD, 45ft T2FD, 28ft vertical, 65ft doublet
HF Receivers: Ten-Tec RX340, RX320, Harris R2368, Cubic R3030A
Decoders: Code300-32, Universal M-8000, PK-232MBX/DSP
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