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Hi everyone.
I'm playing around with RF receiver for HF bands, and I have some trouble with mixer termination.
Let's suppose a classic diode ring (DBM). AFAIK, all ports should be 50 ohm terminated (mainly the IF output). The IF port is easy (using a diplexer). The RF input is again no trouble, if antenna is matched to 50 ohm.
But what about the local oscillator input (LO) to the mixer?
Take, as an example:
http://www.phonestack.com/farhan/dc40.html
The oscillator output is buffered by Q4 emitter follower. According to bias voltages, collector current is close to 6 mA (6V across 1 kohm to ground), and the output impedance can be approximated by Vt/Ic = 4.3 ohm. This is far from 50 ohm.
Also, there're receivers which use an RF preamplifier between the antenna/filter and the mixer RF input, or and active filter like:
http://www.qrp4u.de/docs/en/40m_rx/
I suppose that the output impedance of this active stage should be 50 ohm too.
In this last example, there's positive feedback (output is shunt sampled via a capacitor) so I suppose it raises the low emitter follower impedance towards 50 ohm.
But, on other schematics I just can't figure out how an rf amplifier (common emitter/source) is made 50 ohm output. Is there any book/resource where I can read about this particular topic?
Thanks!
I'm playing around with RF receiver for HF bands, and I have some trouble with mixer termination.
Let's suppose a classic diode ring (DBM). AFAIK, all ports should be 50 ohm terminated (mainly the IF output). The IF port is easy (using a diplexer). The RF input is again no trouble, if antenna is matched to 50 ohm.
But what about the local oscillator input (LO) to the mixer?
Take, as an example:
http://www.phonestack.com/farhan/dc40.html
The oscillator output is buffered by Q4 emitter follower. According to bias voltages, collector current is close to 6 mA (6V across 1 kohm to ground), and the output impedance can be approximated by Vt/Ic = 4.3 ohm. This is far from 50 ohm.
Also, there're receivers which use an RF preamplifier between the antenna/filter and the mixer RF input, or and active filter like:
http://www.qrp4u.de/docs/en/40m_rx/
I suppose that the output impedance of this active stage should be 50 ohm too.
In this last example, there's positive feedback (output is shunt sampled via a capacitor) so I suppose it raises the low emitter follower impedance towards 50 ohm.
But, on other schematics I just can't figure out how an rf amplifier (common emitter/source) is made 50 ohm output. Is there any book/resource where I can read about this particular topic?
Thanks!
