[minima] Re: A (temporary) solution to my mixer problems
- From: "Wayne" <nb6m@xxxxxxx>
- To: <minima@xxxxxxxxxxxxx>
- Date: Wed, 22 Oct 2014 14:35:18 -0700
Hi Joe,
Just re-did the test, on the Rigol DSA-815.
Raw Si570 output +14.47 dBM
With 39 Ohm resistor +10.37 dBm
So yes, it does drop the output significantly, more than the scope and power
meter suggested.
But it looks like mileage will vary, so folks should still measure the output
after putting the 39 Ohm resistor in and adjust as necessary to get it down to
+7 dBm.
Thanks a bunch. I appreciate the double check.
Wayne
From: Joe Rocci
Sent: Wednesday, October 22, 2014 2:17 PM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: A (temporary) solution to my mixer problems
Wayne
I just did the test on a spectrum analyzer:
1) Raw output of Si570 = 13.5 dbm
2) After 43 ohm resistor (I didn't have a 39 handy), approx 8 dbm.
How are you measuring RMS voltage and power? Most inexpensive instruments I
know of have problems with complex waveforms, ie, anything that isn't a sine
wave. Square waves drive most of them nuts. This is also true of power meters
that use the Analog Devices log chips.
Joe
W3JDR
-------- Original Message --------
Subject: [minima] Re: A (temporary) solution to my mixer problems
From: Wayne McFee <nb6m@xxxxxxx>
Date: Wed, October 22, 2014 4:04 pm
To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
Joe,
I agree with using the 39 Ohm resistor to get to 50 Ohms, but in tests I've
just run here, that only dropped the output into a 50 Ohm load by some two dBm.
Right now I'm seeing .985V RMS output on my scope, which correlates to just
about +13 dBm, and that's with the 39 Ohm resistor in place.
I'm also seeing +13 dBm on my power meter, which backs up the voltage reading.
This indicates that a resistive pad would still be needed to get the output
down to the +7 dBm level.
Have you run similar tests there?
Thank you,
Wayne
On Oct 22, 2014, at 11:37 AM, Joe Rocci <joe@xxxxxxxxxx> wrote:
The Si570 drives about +13 dbm into a 50 ohm load and its output impedance
is around 13 ohms. With that assumption, all you need is a series 39 ohm
resistor to the mixer LO input. This will create a nice impedance match for the
mixer and will also drop the Si570 drive level just about into the sweet spot.
Joe
W3JDR
----- Original Message -----
From: Wayne McFee
To: minima@xxxxxxxxxxxxx
Sent: Wednesday, October 22, 2014 2:23 PM
Subject: [minima] Re: A (temporary) solution to my mixer problems
Dan,
That's right, it is simply PI network attenuator consisting of three
resistors, with two going to ground and one bridging the top ends of the two.
Like so:
In - rrrrrrr - Out
r r
r r
r r
G G
There are PI network calculators on line which will tell you what value
resistors to use.
In my case a 10 dB attenuator was needed, so I used two 100 Ohm resistors
to ground and a 68 Ohm resistor between the two.
Hope that helps.
Wayne
On Oct 22, 2014, at 11:02 AM, Mvs Sarma <mvssarma@xxxxxxxxx> wrote:
hi Dan,
IMHO , is conventional to call T or Pi attenuator a pad.(three
resistors forming a T attenuator or pi attenuator.)
On Wed, Oct 22, 2014 at 11:16 PM, Dan Reynolds <on30ng@xxxxxxxxx> wrote:
Wayne, for us dummies can you show us what the pad looks like -
schematic. Just need a brain check.
73
Dan -- KB9JLO
https://sites.google.com/site/on30ng/
On Wed, Oct 22, 2014 at 12:05 PM, Wayne McFee <nb6m@xxxxxxx> wrote:
Thomas,
I'm into a second build of the Minima and am using an ADE-1 as the
first mixer as well.
Since the Si570 output is some +13 dBm, and the ADE-1 needs only
+7dBm LO drive, I placed a 50 Ohm resistive pad in line to reduce it down to
that level.
If you haven't yet done that, you should as that will help reduce
spurious responses from the mixer and should reduce LO leakage on transmit as
well.
I have just the receive portion built so far, so cannot yet give
you ye figures from mine.
73,
Wayne NB6M
> On Oct 21, 2014, at 9:42 PM, Thomas Sarlandie
<thomas@xxxxxxxxxxxxx> wrote:
>
> Gang,
>
> After looking at the RF386 this afternoon I came to the
conclusion that my only problem was my Minima. With -30 dBm on the output my
signal was just too low, even for the RF386.
>
> I looked into the recent discussions about mixers and wanted to
build one of them but I do not have the IC you discussed and I could not find a
clear call on which one of the JFET mixers was the best.
>
> So I took a step back and realized that (to me) this project is
more about building a working radio than running lots of mixers experiment. I
completely removed the KISS mixer from my Minima and replaced it with one of
the ADE-1 mixer that I had ordered a few months ago.
>
> My output is now at -15dBm @14.250 and -26@34.25.
>
> <dsa 201410211956.png>
>
> I should now have plenty enough power to drive the RF386!
>
> I think we should recommend this solution to new builders who are
just getting started, at least until we have a good mixer that can easily and
repeatability be built with good performance.
>
> I paid $15 for 4 of them on eBay. $3.5 a piece.
>
> Thoughts?
>
> 73,
> thomas
--
Regards
Sarma
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