At 5/12/2006 09:16, you wrote:
>Re: Hamtronics 224MHz Repeater Kits
>
>The receiver squelch is also mushy, part of the hamtronic rx
>circuit, which is based pretty much out of the Motorola Data
>Book for the MC-3357 or MC-3359 chip they used at the time. I
>know they're using a different receiver combo chip in the newer
>generation receivers, so you'd have to look up the data sheet
>to begin changing the squelch circuit around. Our good friends
>at Spectrum Actually have/had the best squelch circuit for the
>above name chips.
I'm familiar with that IC. IMO it has no "squelch circuit" per se, unless
you call a dedicated audio amp & mute switch a squelch circuit. At any
rate, Hamtronics' noise filter implementation had poor transient response,
which seems to account for why it would seem "tight" whenever any kind of
impulse noise was present on the input (radar, ignition or power line noise).
>I also had put an additional helical resonator preamp on the
>front of the receiver to help protect the front end.
I never had any overload problems with my 2 meter Hamtronics RX, but then
again it never made it into any real comm site. If it were to need more
protection, I'd add a low loss pass cavity instead of a preamplified
helical resonator. The noise figure of a preamp'd helical assembly (hence
your RX system) is equal to the loss of the helical resonator plus the
preamp NF.
> At the
>time, Hamtronics was selling a preamp backed helical front end,
>which I liked quite a bit for the high IMD protection of the
>bipolar (if memory serves me right) device. Now they offer a
>GasFet version and I have no idea what the 3rd order values are
>for the GasFet Circuit.
Every GaAsFET I've measured (maybe 3?) had a 1 dB GCP of +7 dBm
> You could also order (if still available)
>the Toko Helical Filters direct from Digi-Key and "roll your
>own" if you want the more hardy bipolar device or an even better
>Phempt.
Filtering follows the "2 out of 3" rule: small, high selectivity, low loss
- pick any 2. When I have the room, I prefer to use pass cavities instead
of helical filters. That way I can have both high dynamic range & low
noise figure.
Bob NO6B
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