#### Si 5351 output power

barry halterman

Does anyone know if the SI clock chip has enough oomph to drive a 7dbm mixer? I don't want to let the magic smoke out of the only one I have.
Barry
K3bo

iz oos

I don't know the phase noise either

Il mar 20 ott 2020 07:06 PM barry halterman <kthreebo@...> ha scritto:
Does anyone know if the SI clock chip has enough oomph to drive a 7dbm mixer? I don't want to let the magic smoke out of the only one I have.
Barry
K3bo

Gavin Jacobs

The Si5351 has a square wave output at 3.3V, but each clock can be programmed to a different 'drive level'. The choices are 2|4|6|8 mA. Search for Si5351 AN619 to see the (meagre) details.

Gordon Gibby

The 3.3V output isn't at 50 ohms.    You'll get a lot less at 50 ohms.   I think the output impedance can be approximated as somewhere around 500-1000 ohms (off the top of my head).   You'll get about 1.4 Vrms (top of my head) across 500 ohms.   That means you get about 0.140 Vrms across 50 ohms if you use a voltage divider.   Square that, divide by 50, the take the base 10 log, and multiply by 10 and you'lll have the dBm you might expect.

Cheers,
Gordon

On Tue, Oct 20, 2020 at 7:19 PM Gavin Jacobs <apriljunkbox@...> wrote:
The Si5351 has a square wave output at 3.3V, but each clock can be programmed to a different 'drive level'. The choices are 2|4|6|8 mA. Search for Si5351 AN619 to see the (meagre) details.

Gordon Gibby

I think its going to be seriously lower than 7dBm.

To work it the other way, calculate how many milliwats is 7dBm, then calculate the RMS voltage of that dissipation in a 50 ohm load, and compare to the 50 ohm output voltage of a si5351.

The first time I took one and ran it into a 50 ohm load I was a bit shocked.    I think these are a cmos output....not intended to be a power transmitter.   I may be off a bit, so someone with a scope at hand can try it and find out or read the spec sheet more closely.

Cheers,
Gordon

On Tue, Oct 20, 2020 at 8:45 PM Gordon Gibby <docvacuumtubes@...> wrote:
The 3.3V output isn't at 50 ohms.    You'll get a lot less at 50 ohms.   I think the output impedance can be approximated as somewhere around 500-1000 ohms (off the top of my head).   You'll get about 1.4 Vrms (top of my head) across 500 ohms.   That means you get about 0.140 Vrms across 50 ohms if you use a voltage divider.   Square that, divide by 50, the take the base 10 log, and multiply by 10 and you'lll have the dBm you might expect.

Cheers,
Gordon

On Tue, Oct 20, 2020 at 7:19 PM Gavin Jacobs <apriljunkbox@...> wrote:
The Si5351 has a square wave output at 3.3V, but each clock can be programmed to a different 'drive level'. The choices are 2|4|6|8 mA. Search for Si5351 AN619 to see the (meagre) details.

Evan Hand

Gordon,
According to the datasheet from Silicone Labs the output impedance is 50ohms.  Page 8 I have copied here:

73
Evan
AC9TU

Gordon Gibby

Interesting.   Give it a try.   I built my own board and tried to use it to drive an SWR meter.....very unsuccessfully.    That drives my pessimism.   That spec says it is a "DC" spect....that might be an important issue....dunno.   Gordon

On Tue, Oct 20, 2020 at 8:52 PM Evan Hand <elhandjr@...> wrote:
Gordon,
According to the datasheet from Silicone Labs the output impedance is 50ohms.  Page 8 I have copied here:

73
Evan
AC9TU

Evan Hand

Gordon,
What power level did you specify in the programming?  There are a number available and if set for a lower value would not drive the output to full voltage.

73
Evan
AC9TU

Evan Hand

I found this in the Arduino code:

For the v5, line #54 sets the drive level to 8ma, the max that is available.  If not set, I would assume that it would be the lower 2ma value.

73
Evan
AC9TU

Gary Anderson

The Si IBIS model, I found when we were discussing this +2 years ago.
https://www.silabs.com/documents/public/software/si5351a-b-gt.ibs
Hopefully this will be of some assistance.
Rgds,
Gary

Jerry Gaffke

Even at 8ma and with a series cap, the Si531 driver can't drive a 50 ohm load.
With a 3v square wave centered on ground, current peaks into 50 ohms would be
1.5v/50=30ma.  And as the Si5351 gets loaded down, you get significant crosstalk
between the three outputs.

I can, however, believe that the Si5351 has a 50 ohm source impedance, as stated
in the datasheet.  If so,current peaks when driving a 50 ohm load would be
1.5v/(50+50) = 15ma.  Still more than the 8ma Si5351 driver can deliver.

Digital clock drivers often use series termination.  In this case, this could
be modeled by a 50 ohm series resistor at the output of a square wave voltage source.
Digital clock drivers are typically driving the capacitive loads of one or more input buffers
on chips several inches away.  The reflection from the load bounces back to
the driver, where it is absorbed by the 50 ohm resistor.  Though the impedance of
the trace might be over 100 ohms, the 50 ohm series resistor still helps
considerably to dampen the reflection.  Without the resistor, the resultant
ringing can cause false clock triggers in the target chip.

A 7dBm mixer will work with less than 7dBm of drive, though extremely large signals
will be more distorted if the local oscillator is not quite strong enough.
I suspect Farhan has spent plenty of time making these tradeoffs, a good bet
for driving a 7dBm mixer would be to copy what was done on the uBitx.

Jerry, KE7ER

Ashhar Farhan

The ubitx mixers are 7 dbm mixers. So, it does work at 7 dbm drive. The actual output impedance as I measured it with the output set to 8ma was about 80 ohms. Just check the ubitx circuit.
1. It is vital to have a capacitor between the clock output and diode mixer.
2. Use an attenuator pad, I used 6 db pad, between the diode mixer and the si5351. This provides better termination to the mixer as well as the Si5351.
- f

On Wed 21 Oct, 2020, 8:00 AM Jerry Gaffke via groups.io, <jgaffke=yahoo.com@groups.io> wrote:
Even at 8ma and with a series cap, the Si531 driver can't drive a 50 ohm load.
With a 3v square wave centered on ground, current peaks into 50 ohms would be
1.5v/50=30ma.  And as the Si5351 gets loaded down, you get significant crosstalk
between the three outputs.

I can, however, believe that the Si5351 has a 50 ohm source impedance, as stated
in the datasheet.  If so,current peaks when driving a 50 ohm load would be
1.5v/(50+50) = 15ma.  Still more than the 8ma Si5351 driver can deliver.

Digital clock drivers often use series termination.  In this case, this could
be modeled by a 50 ohm series resistor at the output of a square wave voltage source.
Digital clock drivers are typically driving the capacitive loads of one or more input buffers
on chips several inches away.  The reflection from the load bounces back to
the driver, where it is absorbed by the 50 ohm resistor.  Though the impedance of
the trace might be over 100 ohms, the 50 ohm series resistor still helps
considerably to dampen the reflection.  Without the resistor, the resultant
ringing can cause false clock triggers in the target chip.

A 7dBm mixer will work with less than 7dBm of drive, though extremely large signals
will be more distorted if the local oscillator is not quite strong enough.
I suspect Farhan has spent plenty of time making these tradeoffs, a good bet
for driving a 7dBm mixer would be to copy what was done on the uBitx.

Jerry, KE7ER

Gordon Gibby

Thanks, guys!   Experimental data always beats theory!

Gordon

On Oct 20, 2020, at 23:07, Ashhar Farhan <farhanbox@...> wrote:

﻿
The ubitx mixers are 7 dbm mixers. So, it does work at 7 dbm drive. The actual output impedance as I measured it with the output set to 8ma was about 80 ohms. Just check the ubitx circuit.
1. It is vital to have a capacitor between the clock output and diode mixer.
2. Use an attenuator pad, I used 6 db pad, between the diode mixer and the si5351. This provides better termination to the mixer as well as the Si5351.
- f

On Wed 21 Oct, 2020, 8:00 AM Jerry Gaffke via groups.io, <jgaffke=yahoo.com@groups.io> wrote:
Even at 8ma and with a series cap, the Si531 driver can't drive a 50 ohm load.
With a 3v square wave centered on ground, current peaks into 50 ohms would be
1.5v/50=30ma.  And as the Si5351 gets loaded down, you get significant crosstalk
between the three outputs.

I can, however, believe that the Si5351 has a 50 ohm source impedance, as stated
in the datasheet.  If so,current peaks when driving a 50 ohm load would be
1.5v/(50+50) = 15ma.  Still more than the 8ma Si5351 driver can deliver.

Digital clock drivers often use series termination.  In this case, this could
be modeled by a 50 ohm series resistor at the output of a square wave voltage source.
Digital clock drivers are typically driving the capacitive loads of one or more input buffers
on chips several inches away.  The reflection from the load bounces back to
the driver, where it is absorbed by the 50 ohm resistor.  Though the impedance of
the trace might be over 100 ohms, the 50 ohm series resistor still helps
considerably to dampen the reflection.  Without the resistor, the resultant
ringing can cause false clock triggers in the target chip.

A 7dBm mixer will work with less than 7dBm of drive, though extremely large signals
will be more distorted if the local oscillator is not quite strong enough.
I suspect Farhan has spent plenty of time making these tradeoffs, a good bet
for driving a 7dBm mixer would be to copy what was done on the uBitx.

Jerry, KE7ER

Jerry Gaffke

Let's assume the source impedance of the Si5351 is 80 ohms,
and the Si5351 can internally generate a 3v pk to pk square wave, so 1.5vrms.
The current through a 50 ohm load is:    i=1.5/(80+50)=0.01154 amps rms.

That's a bit more than the AN619 value of 8ma, but in the ballpark.
I would guess that evaluating the source impedance using a lighter load
will show the source impedance dropping toward 50 ohms.
By varying the load resistor and measuring the voltage across it, we may
come up with consistent numbers for the source impedance and the voltage source.
I would do this first at fairly low frequencies, then see if it remains true at
the frequency of interest where things may get considerably more complicated.

Power into the 50 ohm load is:    i*i*r = 0.01154*0.01154*50 = 0.006659 watts, or 6.659mw
In dBm, that would be:    10*log10(6.659) = 8.23
With a 6dB attenuator, the 7dBm mixer is seeing 8.23-6 = 2.23 dBm
That can work fine, though might be have noticeable distortion with very strong signals.

There may be variation between Si5351's.
If some truly do limit current to 8ma, we get:
mw=0.008*0.008/50= 0.0032 watts,   10*log10(3.2)=5.05dbm,  mixer gets 5.05-6 = -0.5dBm

The uBitx v5 and v6 have an extra 33 ohm resistor between the Si5351 and the 6dB attenuator,
That would raise the effective Si5351 source impedance from 80 to 103 ohms, and give around 9ma.
(Or perhaps it raises the typical source impedance from 50 to 83 ohms?)
Since the Si5351 is not being asked to work so hard, it will distort less.

The fact that Farhan has found we need an attenuator between the Si5351 and the mixer
suggests to me that the source impedance of the Si5351 is not truly 50 ohms.
Without an attenuator, mixer products would come out of the mixer toward the Si5351,
and be reflected back into the mixer to mix again, creating birdies in the receiver.
I assume this was evaluated with 50 ohm coax between the Si5351 and the mixer,
or with extremely short wires.

Note that I have lots of "may", "could", "think" and "perhaps" in the above text.
Gordon is correct, this sort of armchair engineering only goes so far.
I really should do my own measurements of the Si5351 source impedance.
But I'd bet that Farhan has arrived at a good solution on the uBitx.
It keeps the circuit simple, avoiding a buffer amp between the Si5351 and the attenuator.
And does what it must to quiet down the unwanted mixer products.

Jerry, KE7ER

barry halterman

Thanks guys for all the input on this subject. I have a project that I want to build that utilize the si5351 to drive a SBL-1 or TUF-1 mixer. Will start the build.
Thanks
Barry
K3bo

On Wed, Oct 21, 2020, 11:02 AM Jerry Gaffke via groups.io <jgaffke=yahoo.com@groups.io> wrote:
Let's assume the source impedance of the Si5351 is 80 ohms,
and the Si5351 can internally generate a 3v pk to pk square wave, so 1.5vrms.
The current through a 50 ohm load is:    i=1.5/(80+50)=0.01154 amps rms.

That's a bit more than the AN619 value of 8ma, but in the ballpark.
I would guess that evaluating the source impedance using a lighter load
will show the source impedance dropping toward 50 ohms.
By varying the load resistor and measuring the voltage across it, we may
come up with consistent numbers for the source impedance and the voltage source.
I would do this first at fairly low frequencies, then see if it remains true at
the frequency of interest where things may get considerably more complicated.

Power into the 50 ohm load is:    i*i*r = 0.01154*0.01154*50 = 0.006659 watts, or 6.659mw
In dBm, that would be:    10*log10(6.659) = 8.23
With a 6dB attenuator, the 7dBm mixer is seeing 8.23-6 = 2.23 dBm
That can work fine, though might be have noticeable distortion with very strong signals.

There may be variation between Si5351's.
If some truly do limit current to 8ma, we get:
mw=0.008*0.008/50= 0.0032 watts,   10*log10(3.2)=5.05dbm,  mixer gets 5.05-6 = -0.5dBm

The uBitx v5 and v6 have an extra 33 ohm resistor between the Si5351 and the 6dB attenuator,
That would raise the effective Si5351 source impedance from 80 to 103 ohms, and give around 9ma.
(Or perhaps it raises the typical source impedance from 50 to 83 ohms?)
Since the Si5351 is not being asked to work so hard, it will distort less.

The fact that Farhan has found we need an attenuator between the Si5351 and the mixer
suggests to me that the source impedance of the Si5351 is not truly 50 ohms.
Without an attenuator, mixer products would come out of the mixer toward the Si5351,
and be reflected back into the mixer to mix again, creating birdies in the receiver.
I assume this was evaluated with 50 ohm coax between the Si5351 and the mixer,
or with extremely short wires.

Note that I have lots of "may", "could", "think" and "perhaps" in the above text.
Gordon is correct, this sort of armchair engineering only goes so far.
I really should do my own measurements of the Si5351 source impedance.
But I'd bet that Farhan has arrived at a good solution on the uBitx.
It keeps the circuit simple, avoiding a buffer amp between the Si5351 and the attenuator.
And does what it must to quiet down the unwanted mixer products.

Jerry, KE7ER

James Lynes

Look at Charlie's(ZL2CTM) videos. He has good results directly driving SBL-1s and ADE-1s from the si5351.

James

barry halterman

Charlie's  sdr , 9 part videos?
Barry

On Wed, Oct 21, 2020, 11:52 AM James Lynes <jmlynesjr@...> wrote:
Look at Charlie's(ZL2CTM) videos. He has good results directly driving SBL-1s and ADE-1s from the si5351.

James

James Lynes

He has several video series using the si5351. Unfortunately there's not an index. He also has a blog.

James