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Mixer and filter impedance matching




 
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Author Topic: Mixer and filter impedance matching  (Read 327 times)
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KI4YAN
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« on: May 13, 2018, 12:32:00 PM »

I've been piddling with a little solid state rig for a while now. Both the LO and VFO are synthesized and stable, and the mixers work *fantastically*, with IP3 performance greater than +37dB on the bands of interest. (H-mode mixer)

I've been using a bidirectional IF strip, switching the oscillators and IF amplifiers to change the direction. It works, but the IF amps are not great, only really showing +11dB IP3 performance. Also I'm told that I only need one IF amp, and I have no AGC happing in the IF. Also, the crystal filter matching is not great.

To get the really stellar performance out of the mixers, I am using a diplexer to present a stable 50R load on the IF ports. The problem comes when it's time to do the impedance transform to feed the filter-I need to transform that 50R load to a 1000R+25pf filter.

Currently, I am using a two-transistor amplifier that I am not sure does the job properly. Hopefully the silly black background image that Eagle produces is visible.



Now, this IF amp is SUPPOSED to present a 50R input impedance, and a 1000R output impedance, and be relatively insensitive to reflected impedances. Gain shouldn't be high-I only really want about 8dB-10dB, just enough to overcome the mixer loss (4dB), and the crystal filter loss (roughly 2.5dB when matched properly).

Can anyone recommend a better method, or maybe a better IF amplifier? One that implements AGC?
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W1RKW
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« Reply #1 on: May 13, 2018, 01:13:36 PM »

if the output impedance of the preceding stage is high to a relatively low input impedance of the next stage,  it can have a loading effect on the previous stage that affects the frequency response and output signal level of that stage.  Some sort of preceding buffer stage might be in order if this input and output impedance configuration  is needed.
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Bob
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His fear was when I turned it on for the first time life on earth would come to a stand still.
KI4YAN
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« Reply #2 on: May 13, 2018, 04:18:55 PM »

Well, the above IF amplifier presents a 50R input at T3's (the leftmost) base. What is currently feeding it is the 9Mhz diplexer, which, is used to terminate the IF port of the mixer and provide a 50R impedance for the mixer's IF port. Without the good 50R match, both at and away from the 9mhz IF, the mixer's performance degrades quickly.

I guess what I am asking is does a diplexer care what impedance it's terminated by? If the diplexer can prevent the mixer IF port from seeing the 1000R input impedance of the filter, will it matter?
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #3 on: May 13, 2018, 05:09:51 PM »

Use a J-Fet J310 in common base with a source resistor to adjust the drain current at 10 - 15 mA. When you select a FET with a high transconductance (close to 20 mA/V),, you are pretty close to 50 Ohms in without phase problems. In the drain a parallel LC circuit with a 1 kOhm damping resistor  (e.g. small ring core inductor) and you are fine, Good IMD and simple
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