[R-390] Filters

[Bob Camp]

Hi

You have multiple filters in the radio. You can indeed loose all of them at once. From what I've seen that's relatively rare. The most likely replacement situation would be swapping out one or two and leaving the rest in the radio. That puts some limits on the insertion loss and bandwidth of the filters. If I already have a working 8 KHz wide filter in the radio, there's no need to replace the 4 KHz filter with a 8 KHz L/C filter.

I still believe that we are a long way from radios that have lost all their filters from wear out.  There are still some questions to be answered about why the 4 KHz filter fails. Figuring that out is the real key to all this.

Bob 


On Feb 20, 2013, at 12:09 AM, Drew P. <drewrailleur807 at yahoo.com> wrote:

> Hi Bob,
> 
> Thank you for your input.
> 
> "Wide" and "narrow" to me are relative terms, and just about anything will be narrower than a jumpered-out defunct mechanical filter.  In the approach which I mentioned, I would not be striving for performance to equal that of the mechanical filters, but rather, something reasonably good (to me).  Some would want replacements to equal or surpass the performance of the original filters; I respect their position. Me?  I say, if it's better than bad, its good.
> 
> Drew 
> 
> --- On Tue, 2/19/13, Bob Camp <ham at kb8tq.com> wrote:
> 
>> From: Bob Camp <ham at kb8tq.com>
>> Subject: Re: [R-390] Filters
>> To: "Drew P." <drewrailleur807 at yahoo.com>
>> Date: Tuesday, February 19, 2013, 7:40 AM
>> Hi
>> 
>> I've seen narrow 455 KHz L/C filters with 50 db of insertion
>> loss…
>> 
>> You will need to do some significant impedance transforms to
>> get the modern filters into an R-390. There's likely to be
>> some loss there. Having some loss already in the signal
>> chain should make that all a bit easier.
>> 
>> Bob
>> 
>> On Feb 19, 2013, at 12:01 AM, Drew P. <drewrailleur807 at yahoo.com>
>> wrote:
>> 
>>> 
>>> Bob Camp wrote:
>>> [snipped]
>>> 
>>> "A narrow mechanical will have less insertion loss than
>> a narrow LC filter (higher Q counts). With a gain stage or
>> two you could fix that. It would make straight drop in a bit
>> more complex."
>>> 
>>> IIRC, the Collins mechanical filters used in the R-390A
>> have a very high insertion loss, even when working as
>> designed.  This would be a reason why they were placed
>> AFTER the first 455KHz IF stage.  If these filters
>> indeed have high loss, then perhaps no additional gain stage
>> would be required when substituting a many pole LC filter
>> for mechanical. Lossy original filter, lossy replacement.
>>> 
>>> Drew
>>> 
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>>