#### Si570 LVDS Output Impedance

Alfredo Mendiola Loyola

Hello,

If I connect a Center Tap of an 1:4 Transformer (Secondary) to the LVDS outputs I will get a 50 Ohm output impedance on the Primary?

Schematic:

I understand that the output impedance of each output of the si570 is 100 Ohm (LVDS), When I connect each output of the si570 to the transformer Secondary I will get 200 ohm and 50 ohm on the Primary (200 4 = 50 ohm).

Do I have to connect the center tap of the secondary to ground?

Transformer Datasheet:
https://www.coilcraft.com/pdfs/pwb.pdf
Model: PWB-4-BL

73
Alfredo Mendiola
OA4AJP

Clifford Heath

Check that you get the right impedance ratio - remember that it is the
square of the turns ratio.

I haven't checked the data sheet to see if the Si570 output is DC (ground)
referenced. If not, I would decouple the CT to ground with a capacitor
anyway. But check the data sheet.

Clifford Heath

On Mon, 22 Apr 2019, 02:29 Alfredo Mendiola Loyola via Groups.Io
<mendiola_loyola=yahoo.com@groups.io> wrote:

Hello,

If I connect a Center Tap of an 1:4 Transformer (Secondary) to the LVDS
outputs I will get a 50 Ohm output impedance on the Primary?

Schematic:

I understand that the output impedance of each output of the si570 is 100
Ohm (LVDS), When I connect each output of the si570 to the transformer
Secondary I will get 200 ohm and 50 ohm on the Primary (200 4 = 50 ohm).

Do I have to connect the center tap of the secondary to ground?

Transformer Datasheet:
https://www.coilcraft.com/pdfs/pwb.pdf
Model: PWB-4-BL

73
Alfredo Mendiola
OA4AJP

Clifford Heath

The Si570 data sheet says: "Available LVPECL, CMOS, LVDS, and CML outputs".
So the correct output coupling depends on which one you use.

LVPECL (fastest) requires 50 ohms termination to Vdd-2.0V.
LVDS and CML require 100 ohm differential (not supply-referenced).
CMOS swings within about 0.4V of the rails.

Clifford Heath.

On 22 Apr 2019, at 12:39 pm, Clifford Heath via Groups.Io <clifford.heath=gmail.com@groups.io> wrote:

Check that you get the right impedance ratio - remember that it is the
square of the turns ratio.

I haven't checked the data sheet to see if the Si570 output is DC (ground)
referenced. If not, I would decouple the CT to ground with a capacitor
anyway. But check the data sheet.

Clifford Heath

On Mon, 22 Apr 2019, 02:29 Alfredo Mendiola Loyola via Groups.Io
<mendiola_loyola=yahoo.com@groups.io> wrote:

Hello,

If I connect a Center Tap of an 1:4 Transformer (Secondary) to the LVDS
outputs I will get a 50 Ohm output impedance on the Primary?

Schematic:

I understand that the output impedance of each output of the si570 is 100
Ohm (LVDS), When I connect each output of the si570 to the transformer
Secondary I will get 200 ohm and 50 ohm on the Primary (200 4 = 50 ohm).

Do I have to connect the center tap of the secondary to ground?

Transformer Datasheet:
https://www.coilcraft.com/pdfs/pwb.pdf
Model: PWB-4-BL

73
Alfredo Mendiola
OA4AJP

Ambro IW2FVO

https://www.sdr-kits.net/documents/PA0KLT_Manual.pdf

and look at fig. 7 on page 13/21 for impedance / transformer connections.

I am using the Si570 as a VFO for my airband AM receiver too with good results. I did try it last week !
Used also for : Two meters FM receiver and for APT (137 MHz ) 40 KHz IF FM receiver.
all the best !
73, iw2fvo
Ambrogio

Alfredo Mendiola Loyola

Hello:

My goal is to get a single channel output with exactly 50 ohms impedace.

By using a transformer 1:4 if the outputs of the si570 doesn't have exactly 50 ohm I will get at the output of the transformer an impedace higher or lower than 50 ohm.

I'm working with UHF sgnales 430 mhz.

73
Alfredo.
OA4AJP

Nick Kennedy

For that purpose, I'd try to assure 50 ohms by following the output with an
attenuator. I wouldn't assume a specific source resistance internal to the
Si570.

Another issue if you're trying to use the si570 in a signal generator type
of application is that the Si570 has a square wave output. However, if as
you say you are interested in a single channel use, you can LPF that signal
into a sine wave.

73-

Nick, WA5BDU

On Mon, Apr 22, 2019 at 6:28 AM Alfredo Mendiola Loyola via Groups.Io
<mendiola_loyola=yahoo.com@groups.io> wrote:

Hello:

My goal is to get a single channel output with exactly 50 ohms impedace.

By using a transformer 1:4 if the outputs of the si570 doesn't have
exactly 50 ohm I will get at the output of the transformer an impedace
higher or lower than 50 ohm.

I'm working with UHF sgnales 430 mhz.

73
Alfredo.
OA4AJP

Alfredo Mendiola Loyola

Hello,

I will use a 1:4 transformer with center tap to connect the output of the si570 (LVDS).
The secondary of the transformer (Center Tap) will be connected to the differential outputs of the si570
The primary of the transformer will be connected to a 3dB PI attenuator to get 50 ohm at the input and output of the attenuator.

The si570 datasheet states that the LVDS model has an output of 0.7 vpp {swing (diff)}

If I use a 1:4 transformer (Coilcraft PWB-4-BL) what will be the output peak to peak voltage at the transformer Primary?

73.
Alfredo Mendiola Loyola
OA4AJP

Clifford Heath

On 23 Apr 2019, at 11:12 am, Alfredo Mendiola Loyola via Groups.Io <mendiola_loyola=yahoo.com@groups.io> wrote:
I will use a 1:4 transformer with center tap to connect the output of the si570 (LVDS).
The secondary of the transformer (Center Tap) will be connected to the differential outputs of the si570
The primary of the transformer will be connected to a 3dB PI attenuator to get 50 ohm at the input and output of the attenuator.

The si570 datasheet states that the LVDS model has an output of 0.7 vpp {swing (diff)}

If I use a 1:4 transformer (Coilcraft PWB-4-BL) what will be the output peak to peak voltage at the transformer Primary?
1:4 impedance is 1:2 voltage ratio. 0.7Vp-p would reduce by 2:1 to 0.35Vp-p, which is 0.12Vrms.
Into 50 ohms that's about -5dBm.

Clifford Heath.