Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Polarity of this PIN diode in TX mode?

Status
Not open for further replies.

yo2550

New Member
Originally, before I removed it, it was placed opposite to the ground but when I read the schematic diagram it reads pointing to ground.

I broke one of its leg so I replaced it as it was placed originally (opposite to ground). According to its technical description, D8 was placed in parallel to C4 which is a part of T matching network.

So please advise me which polarity of this diode (D8) is correct or should be.

View attachment 66786

Here is the text describing the switch circuit.
E). TRANSMIT/RECEIVE SWITCH
When the radio is in the transmit mode, pin diode switches D13 and D1
are both turned on (representing less than 0.7 ohms). D13 allows the transmit
signal to pass to the antenna and D1 shorts one leg of a T matching network
(L3, L15 and C4) to ground in the receive path. This results in a parallel tuned
circuit high impedance being presented to the transmit signal so that the
receive path does not load the transmit signal. In the receive mode, both D13
and D1 are off, resulting in the antenna signal being coupled into the receive
LNA through the 50 ohm T matching network and the unwanted load of the
transmit final amplifier is reduced to less than 1 pF by D1.
 
Last edited by a moderator:
Your attachment is not here. Instead it is over at the edaboard website.
 
I'm having problems locating any DC voltage source for switching the diodes on :confused: The red boxes don't help in following the circuit either :(. Be that as it may, D8 would seem to be permanently on if it has the polarity shown, so IMO it should be reversed..
 
I noticed this in another thread about this radio a few weeks ago.

There are several things which are odd about that antenna changeover circuit.
I think that there may be some errors in the drawing.

JimB
 
I'm having problems locating any DC voltage source for switching the diodes on :confused: The red boxes don't help in following the circuit either :(. Be that as it may, D8 would seem to be permanently on if it has the polarity shown, so IMO it should be reversed..

Just ignore the red boxes. They are voltages in receiving mode. For your info, the PIN diodes switched on when they are enough current passing it. In this case the rf signal when in trasmitting mode.
 
I noticed this in another thread about this radio a few weeks ago.

There are several things which are odd about that antenna changeover circuit.
I think that there may be some errors in the drawing.

JimB

Thanks. So if the D8 should point to D1, then what would it supposed to function?
 
For your info, the PIN diodes switched on when they are enough current passing it.
In that case they may just have a limiting function. But won't they then be on only during alternate half cycles of the Tx signal? And would it matter which way round they are? I'm still not understanding the circuit :confused:
 
Last edited:
In that case they may just have a limiting function. But won't they then be on only during alternate half cycles of the Tx signal? And would it matter which way round they are? I'm still not understanding the circuit :confused:

D13 allows the transmit
signal to pass to the antenna and D1 shorts one leg of a T matching network
(L3, L15 and C4) to ground in the receive path. This results in a parallel tuned
circuit high impedance being presented to the transmit signal so that the
receive path does not load the transmit signal.

Yes, the PIN diode would be on only half cycle but I'm not sure though because it is VHF. The PIN diode would be on all the way when TX signal is passing it.

The point is I am not sure if the D8 is a part of TX switch that short the transmit signal to ground. So I won't damage the Q6 which I replaced it twice(from the problem of low sensitivity in receiving mode).
 
Yes, the PIN diode would be on only half cycle but I'm not sure though because it is VHF
Why would it being VHF make any difference ?
The point is I am not sure if the D8 is a part of TX switch that short the transmit signal to ground.
You and me both :)
 
That link is broken :(
 
I had previously commented that I thought that the diagram was in error.

I have attached a simplified version to show the wiring without all the component detail clutter.

With the information I have, I still think that there is something wrong with the diagram.

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.


Compare this with the other circuit fragment which I have attached.
It is from an MX294, a VHF FM transceiver made by Philips in the 1980s.

Look at D16 and D17, on transmit they are forward biassed, D16 connects the PA (TR24) to the antenna and D17 shorts out the connection to the receiver input.
On receive, both diodes are un-biassed and so present a high impedance to low level signals.

JimB
 
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:

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!

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
 
Do you have any idea what Q26 is supposed to do in the circuit?
When Q26 turn on it turns on the PIN diodes D7/D13 and D1.

If you follow the line back from the base of Q26, you will see that it is switched by Q13 from the signal TXB+

JimB
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top