Yes, I'd be concerned about non-linear behavior in a 1n4148 up front there
where we have a whole world of RF coming in.
Maybe a single PIN diode?
Though PIN diodes also start becoming non-linear as the frequency falls,
could still have trouble from some blowtorch at 550khz.
The PIN diode bridge's suggested configuration has a 1k resistor on the control voltage,
so that current would be around 1ma at 10v, 0.5ma per diode.? ?
Absolute max forward current on the BAP64Q is 100ma.
I think it's a minor consideration, but the 30mhz LPF may behave differently
at other than 50 ohms.
If you do wind up with a good attenuator controlled by DC,
having DC through the RF gain pot would make wiring it up less critical.
I've got some BAP64Q's on order, will try them after the 12mhz amp.
Had been planning to do this 6 months ago.
Jerry?
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On Tue, Mar 27, 2018 at 06:05 am, Don, ND6T wrote:
After sleeping on it I've got a fresh perspective: Why PIN diodes? We're not at a significant power point. Why a bridge arrangement? At this position we are not concerned with maintaining impedance at idle conditions, they become less important as signals are being attenuated. So, with 1N4148 switching diodes I found that just the two series diodes will impose an idle loss of 3 dB and a control range of 62 dB. The idle current is around 8.3 milliamps. Of course, going to a single diode (things are getting better) I measured an more agreeable 1.4 dB insertion loss with a 6 ma. bias and a control range of 49 dB at 7 MHz, 36 dB at 30 MHz. My principle concerns with using a device in series with the receive path is the generation of IM components, especially when biased for partial conduction. BCI could be a real concern. That was one of the beauties of the shunt transistor.
Tim, the limiting factor of the dynamic range is the finite Rds. Yes, I was surprised also, that I got 20 dB. This is one of the reasons that I question most everything. Sometimes I am rewarded by a boon. At the least I receive an education. Oh, and what article are you referring? -Don
