EMRFD Message Archive 7300

Message Date From Subject
7300 2012-02-28 01:37:14 Ashhar Farhan hybrid twisted pair following the switching mixer
what is the benefit of using a hybrid coupler following a switching
mixer vis a vis using a diplexer? can't a diplexer be used to isolate
the roofing crystal filter from the mixer?

- farhan
7301 2012-02-28 02:11:43 Weddig, Henning-C... Re: hybrid twisted pair following the switching mixer
Farhan,

From my own experiance (designing a high IP frontend):

You can not build a diplexer with a bandwidth narrow enough to match to
tbe bw of an x-tal filter as the values of the diplexer will be not
physically madable!

That is why a FET in common gate configuration with a resitive load on
the drain was used to isolate the high out of band impedance of the
filter and terminate the mixer with a broad band real load.

The diplexer will termiate all higher harmonics of the LO and RF
(according to a personal communication with Watkin Johnson application
engineer up to the fifth order including sums of the signals)


But: the IP of the x-tal filter (for monolithic filters in the range of
+25 dBm!) is much less than the output IP of the mixer and perhaps amp.
The amp will "overdrive" the filter!

Clue: use a diplexer for the higher harmonics, two 90 deg hybrids
designed for the wanted IF (input and output) and two identical x-tal
filters (concerning insertion loss but more important identical
transmission phase) to split and sum the two signals. All mismatching
will be dumped into the resistors of the 90 deg hybrids, resulting that
within the bandwidth of the hybrids You get a "good" match to the mixer.

Regards
Henning Weddig
DK5LV



Am 28.02.2012 10:37, schrieb Ashhar Farhan:
> what is the benefit of using a hybrid coupler following a switching
> mixer vis a vis using a diplexer? can't a diplexer be used to isolate
> the roofing crystal filter from the mixer?
>
> - farhan
>
>
7302 2012-02-29 07:14:38 John Harding Re: hybrid twisted pair following the switching mixer
Hello all,
Are there any opinions on the use of the Balanced amp following the mixer with or without a diplexer? It would seem to resolve the same impedance matching to the mixer without the cost of the second xtal filter set with perhaps a greater imbalance than the FET pair shown in the partial schematic of the Harris RF590 mixer termination. I believe this was also shown in EMRFD, perhaps under advanced receiver concepts. I have not seen it anywhere - homebrew or commercial except in the RF590 - is there a shortcoming with this approach?
Thanks
John


________________________________
7304 2012-02-29 07:22:44 Chris Trask Re: hybrid twisted pair following the switching mixer
>
> Hello all,
> Are there any opinions on the use of the Balanced amp
> following the mixer with or without a diplexer? It would
> seem to resolve the same impedance matching to the mixer
> without the cost of the second xtal filter set with perhaps
> a greater imbalance than the FET pair shown in the partial
> schematic of the Harris RF590 mixer termination. I believe
> this was also shown in EMRFD, perhaps under advanced
> receiver concepts. I have not seen it anywhere - homebrew
> or commercial except in the RF590 - is there a shortcoming
> with this approach?
>

I've been toying with the idea of an active diplexer of sorts, such as using the TL592 with a high-Q series-tuned circuit between the gain adjust pins. I have not seen anything anywhere where anyone has attempted this, but the concept seems reasonable. A low-noise version could be made with a pair of suitable transistors.


Chris