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BITX attenuator

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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
 

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
Rafa? Lichwa?a
 

Hi Peter,

Thanks for reply!

The Bitx is like most things a compromise design (to keep it simple)
Sure. That's clear and even more - that's the whole beauty in this compromise design that it is so simple and it works! :-)
But... even in such simple design, if there is a bug, it should be fixed, right - just to make it working a bit better and still keep it simple.

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.
Are you sure?
My analysis (described in the previous post) shows that crystal filter has 50 ohms Zin/Zout, each transistor amp has impedance translation (so its Zout depends on Zin applied) and mixer has 220 ohms on each port.

Yes the Bitx could be better but then it would become more complex, so
the trade-off.
I'm not suggesting to make it more complex. Let's keep it simple, but try to fix any bugs if they are found anywhere in this design :-)

By the way - I'm still not sure if my previous impedance analysis is correct - can anyone confirm?

The bug found (in my opinion) is just wrong Zin/Zout attenuator between 3rd amp and product detector...

Regards,
Rafal SP3GO
 

Rafal,

You're probably right, the impedance mismatch at the product detector is a good issue to look into. ?Check crystal filter response carefully before and after making any changes to impedance there, the shape will likely get better. ?Perhaps borrow the transformer scheme from the uBitx: ?/g/BITX20/message/22888? ?I'll likely be borrowing the entire uBitx design someday, addresses most of Allison's criticisms and gives multiple bands with a minimum of band specific filter switching. ?I' plan to use the uBitx with a resonant EFHW antenna tuner to limit out of band signals on rx, further reduce harmonics on tx.

Jerry, KE7ER


On Fri, Mar 17, 2017 at 01:45 am, Rafa? Lichwa?a wrote:

The bug found (in my opinion) is just wrong Zin/Zout attenuator between 3rd amp and product detector...

?
 

inexpensive...simply.....yet very functional......mostly great fun!!!!

On Mar 16, 2017 3:30 PM, <pierre@...> wrote:
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

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