DBM and constant R band pass filters.
 

Hello,

I have to use some DBMs in a measurement instrument and of course have to
design the following bp filters.

I have often seen recommended that the DBMs see the resistive load of a
constant R filter (diplexer) rather that the reactive load of a more
traditional filter.

I can think of (model) the DBM mixer to the first order as follows :


.-- +/-1 at LO frequency
|
|
|
V
.------. ZG Zd .-------.
| | ___ ___ | BPF |
VIN--+------------| MULT |---|___|--|___|---+---| H(p) |--- VOUT
| | | | | |
| '------' | '-------'
| ___ .-.
`-|___|----. | |
ZG | | | ZL
.-. '-'
| | |
| |Zd |
| '-' |
I in| | |
| | |
V .-. |
| | |
| |ZL GND
'-'
|
GND
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de


ZG being the source impedance,
Zd the diode dynamic resistance,
ZL the filter input impedance,
H(p) the BPF voltage transfer function.
The multiplier is a high Zin, null Zout device with unity gain.

I can't see any requirement in this first order model for a constant R load.

Do I miss something ? Is it needed because of second order effects ?
Any thought, any pointer to (preferably) detailed analysis is welcomed.

Thanks,
Fred.