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Hi Rafal,

No you have not missed anything and you are not the first to notice the short comings of the design, Allison KB1GMX has also done a very nice analysis of the Bitx design with suggestions as to how it might be improved (of course with some added complexity).

The Bitx is like most things a compromise design (to keep it simple) and yes the single transistor amps and crystal filter have a Zin/Zout of about 200 ohms while the mixers are 50 ohm so there is mismatch and some performance lost.

Also Farham has noted that for better IP3 performance at 40m the first Rx RF amp should probably be disabled but then if you are using your Bitx in the field with a simple wire antenna strung up in a tree the extra gain is probably makes things a little easier.

Yes the Bitx could be better but then it would become more complex, so the trade-off.

Regards,

Peter VK1XP






On 17-03-2017 06:41, Rafa? Lichwa?a wrote:

I plan to build BITX40 TRX, so I've read the project description and
analyze the circuit. As it is well described on the web page, there
are single transistor amplifier stages which have the following
input-output impedance translation:

Rout = (Rfeedback * Remmiter) / Rinput

In all stages there are Rfeedback = 1.1k (two 2.2k in parallel from
both amplifier directions) and Remmiter = 10R.
That means we have the following impedance translation:

Rout = 11000 / Rin

So now, analyzing the circuit starting from the 50R antenna, we have
the following impedances:

antenna: 50R
LPF-input: 50R
LPF-output: 50R
1st AMP-input: 50R
1st AMP-output: 220R (after translation)
diode MIXER(assuming equal impedances on each port): 220R
2nd AMP-input: 220R
2nd AMP-output: 50R (after translation)
Xtal BPF-input: 50R
Xtal BPF-output: 50R
3rd AMP-input: 50R
3rd AMP-output: 220R (after translation)

AND.....

now there is PI-attenuator (-3.85dB) with both 220R shunt to GND and
22R in series.
That means this attenuator is designed for 50R input/output impedance
- it does NOT match 3rd AMP output impedance which is 220R!

Did I miss something or I've just misunderstood the circuit design?

Best regards,
Rafal SP3GO