I have an FM receiver design I wish to implement using the SA605 chip.

The stage before feeding the signal to the SA605 chip is an RF amplifier. I know I have to impedance match between each stage to obtain maximum power transfer, I am just not sure how one goes about in calculating the output impedance of an adjustable RF amplifier stage that uses LC tanks to tune between 88MHz to 108MHz.

My goal is to match the output impedance to the input impedance of the mixer of the chip. Furthermore, I have found online a couple of app notes discussing how to design the impedance network between the output of the RF stage to the input mixer of the SA605 chip but it is in respect to a specific frequency( n SA605.)

How would one select the frequency of the mixer? Note a local oscillator is also mixed with the RF signal, which will be adjusted respectively to maintain a constant 10.7MHz intermediate frequency.

  • 1
    \$\begingroup\$ View your challenge as achieving a large voltage swing on a 10pF load (PCB trace + SA605 Cin) in parallel with thousands of ohms resistance. 1pf at 1GHz is -j159 ohms; 10pF at 100MHz is also -j159 ohms. Thus your load is close to pure capacitance, -j159 ohms. The Q is 20 or 30 or 40, thus your RF (low noise?) amplifier will set the bandwidth. \$\endgroup\$ Mar 1, 2018 at 4:40

1 Answer 1


Your design problem is complicated by requiring tank resonance to vary from 88 - 108 MHz. Variable capacitance diodes might be used. These influence design choices for your variable-frequency tank.
If you go this route, the local oscillator tank resonance must be maintained 10.7 MHz above the RF amplifier tank resonance. Tracking these two resonant circuit frequencies is not easy. So tank bandwidth is often kept fairly wide.

Matching an RF amplifier to SA605 input (about 4700 ohms) is probably not worth the effort. An RF amplifier might have output impedance about a kilo-ohm or two. I'd just use a parallel-resonant tank in between and not worry about matching.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.