Polarity of this PIN diode in TX mode?

[davenn]

OK, I understand the description in section E of the Technical Description document, as far as it goes.

However, it makes no mention of the purpose of D8.
If D8 is not in circuit, it will work as described.
If D8 is in circuit as shown in the schematic, I stand by my previous description of operation:

Yes, looking at the full schematic, there is power on the line which I marked as "PA +ve supply" is energised all the time, it is connected directly to the battery +ve.
(This is common to most radios as it minimises the current requirements of the on/off switch and the PA takes no current when it is not transmitting).

So, as far as your original question goes, I don't know!

JimB

D8 IS shown in the original circuit in the correct orientation. D8 conducts only during transmit and grounds any transmitted signal that gets through the high impedance path via the inductor. It sole purpose is to stop higher voltage RF appearing on the front end of the receiver amplifier Q6.
Yes Jim, the circuit will work without D8, for a while till you finally kill the RX front end

Dave

 

[JimB]

D8 IS shown in the original circuit in the correct orientation. D8 conducts only during transmit and grounds any transmitted signal that gets through the high impedance path via the inductor. It sole purpose is to stop higher voltage RF appearing on the front end of the receiver amplifier Q6.
Yes Jim, the circuit will work without D8, for a while till you finally kill the RX front end

Dave

Yes, but as I pionted out in Post#16 of this discussion:

...

I assume that the PA +ve supply is alive all the time in both transmit and receive modes.**
In that case diodes D7 are forward biassed by current from the PA supply, via L8, L6, L3 and D8 and back to the 0v line.
There is no obvious resistance to limit this current other than the resistance of the diodes and the coils, ie not very much!
On transmit,
I would expect that D7 was forward biassed to create a low resistance path from the PA transistor (Q1) through to
the harmonic filter and the antenna.
I would also expect diodes D1 and D8 to be forward biassed to short out the transmit signal from reaching the receiver RF amplifier.
On receive,
I would expect D7 to be off or reverse biassed to stop the PA from loading the received signal from the antenna.
I would also expect D1 and D8 to be off or reverse biassed to allow the signal through to the receiver RF amplifier.

But, as I pointed out above, D7 and D8 appear to be permanently forward biassed.
So, to get back to yo2550s original question as to whether D8 is drawn the correct way round, I am not sure.
It certainly makes no sense as it is drawn, and I cannot see what good it would do if it were connected the other way round.

** If the PA supply is turned off during receive the switching almost makes sense except for the lack of obvious current limiting on the current through the diodes.

If you follow the supply from the collector of the PA transistor back to the battery, you will see that there is a supply to the collector all the time.
Hence the diodes D17, D13 and D8 are conducting all the time.

JimB

 

[davenn]

Yes, but as I pionted out in Post#16 of this discussion:



If you follow the supply from the collector of the PA transistor back to the battery, you will see that there is a supply to the collector all the time.
JimB

Yes thats pretty common practice for a FM, class C, amplifier

Hence the diodes D17, D13 and D8 are conducting all the time.

Been pondering this for that last hr or so The only thing I can conclude is that because D1,8,7,13 are all PIN, aka Varicap, diodes, is it possible that their capacitance is such that they wont actually conduct DC and only do so in the presence of RF current ?
So during transmit you have a mix of DC bias current and RF current.

This cct is a little different to others in that the others have a cap in series between the collector of the final and the first single or pair of PIN diodes. and the bias is provided by another supply line as in that cct you showed.
Maybe it really is just the type of diode they are using that allows them to work it this way ?

Dave

 

[alec_t]

is it possible that their capacitance is such that they wont actually conduct DC and only do so in the presence of RF current ?

No. For DC the PIN diode behaves much like any general-purpose PN diode.

 

[davenn]

ok no probs
As I said its a little different to most ccts that notmally have a DC blocking cap between the output of the final device and the first PIN diode(s). So far then Im still at a loss to explain its operation if there's DC on the diode all the time


D