[R-390] A 6kc. mechanical filter for a R-390a?

[Al S.]

Thanks for that information on the 390a mechanical filters.

I'll have to have a look at how this 6kc filter is terminated etc in the
Racal 6790 receiver. Its not one of the physically tiny torsional filters.
The only info that I have with the filter is that it needs a 750 pf. cap
for resonating on the input and output of the filter in the circuit.

Also some good reading on mechanical filters from the March 1993 issue of
RF Design magazine. Bill Sabins article is very informative.

Al
On Jun 16, 2017 1:09 AM, "Charles Steinmetz" <csteinmetz at yandex.com> wrote:

> The original Collins filters (the type used in the 390A) are
> self-terminated and are designed to be fed from a Hi-Z source and
> terminated by a Hi-Z load (50k ohms or greater in parallel with 130pF).
> Here is what a contemporary Collins applications note said:
>
> THE DESIGN OF CIRCUITS employing Mechanical Filters is relatively simple,
>> since no special matching networks are normal]y required. Being internally
>> terminated, the filters need only a high-resistance termination (50,000
>> ohms
>> or greater) at either end together with the capacity (approximately 130
>> pf)
>> required to resonate filter input and output at the center frequency.
>>
>> THIS HIGH RESISTANCE is readily obtained by driving the Filter with a
>> pentode tube (effectively a constant current generator) and terminating it
>> into a vacuum tube grid. It was this usage that led to the use of the term
>> "transfer impedance" in specifying the effect of a Mechanical Filter on
>> the
>> gain of a given circuit. The transfer impedance is the ratio of the input
>> current to the output voltage, so the over-all gain of an amplifier stage
>> with a Mechanical Filter following the amplifier tube is simply equal to
>> the
>> transconductance of the tube times the transfer impedance.
>>
>
> Many of the newer rectangular filters are designed to be terminated at
> both ends with 2k ohms in parallel with 30pF.  I am attaching a schematic
> of the Collins test circuit below.  Your filter is most likely of this type.
>
> Best regards,
>
> Charles
>
>
>
> Al S. wrote:
>
> The TM 11-856a Army technical manual for the R-390a says the input and
>> output impedances of the filters are in excess of 20k ohms and resonating
>> capacitances of between 120 to 140 pf. How to measure the impedances
>> accurately.  Maybe its a known value.
>>
>> Al
>>
>>
>> Alan Victor wrote:
>>
>> First you need to establish the current terminating Z for the mechanical
>>> filters in the 390A. Then you need to know the required termination Z for
>>> the ones you wish to install. Designing a matching system to handle this
>>> problem is pretty straight forward. I do not believe the current 390A
>>> mechanical filters are that high a Z. At least the ones I have worked
>>> with
>>> at 455 kHz and built some receivers around were only about 3k//?? Cpf. I
>>> would have to look them up in my notes.
>>>
>>> Alan W4AMV
>>>
>>
>
>
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