[minima] Re: Experimenting with the crystal filter
- From: Thomas Sarlandie <thomas@xxxxxxxxxxxxx>
- To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
- Date: Thu, 23 Jan 2014 02:42:28 -0800
Clark: I had a face-palm moment when I got your email. This is obviously a
much simpler solution. Thanks a lot.
Farhan/Jerry: I read EMRFD on return loss bridge and this page too:
http://www.qrp.pops.net/RF-workbench-3.asp but I am still not sure how I
would measure the output impedance of the dds. If you have some more links
or quick suggestion, that would be very helpful. Thank you.
I used Clark technique to measure the output impedance of the DDS and found
100 ohm again (not a big surprise). I then built an L circuit to match it
to 50 ohms. I did not have the perfect parts available so I ended up with
39+i.14 calculated (66 ohms measured with the pot).
I took the time to graph the filter response:
https://docs.google.com/spreadsheet/ccc?key=0As9CZnZ-A5a2dEZaM1dlWVdVX3pXaUlJMXd0cHNscHc#gid=1
I get a center freq of 20,002.5 kHz, a 3dB bw of 5 kHz and a ripple of 2.2
dB. If you have feedbacks on my method or if you think I goofed somewhere
please feel free to point it out. I will probably try again with matched
crystals ... and a better idea of what I am aiming for.
One thing that still puzzles me is the insertion loss. I get about 10dB
when Mark reported 3 dB. I am not sure what could explain such a big
difference.
thomas
On Wed, Jan 22, 2014 at 2:18 AM, <jerry@xxxxxxxxxxxxxxxx> wrote:
> The return loss bridge is one of the handiest tools on my workbench. I
> got my design from chapter 7 of EMRFD.
>
>
> -------- Original Message --------
> Subject: [minima] Re: Experimenting with the crystal filter
> From: Ashhar Farhan <farhanbox@xxxxxxxxx>
> Date: Wed, January 22, 2014 4:04 am
> To: minima@xxxxxxxxxxxxx
>
> the correct way, is really to measure it with an RLB. six resistors and
> a balun will go a long way in helping us measure these impedances. oh,
> btw, u can measure the input and output impedances with the RLB.
>
> - f
>
>
>
> On Wed, Jan 22, 2014 at 3:08 PM, Clark Martin <cmmac@xxxxxxxxx> wrote:
>
>
> Sent from an iPhone, don't ask whose.
>
> > On Jan 22, 2014, at 1:34 AM, Thomas Sarlandie <thomas@xxxxxxxxxxxxx>
> wrote:
> >
> > I am using an ad9850 and I measured the output impedance to be 100
> ohms. I simply measured the output voltage in open circuit (519 mV rms)
> and the output current in short circuit (5.1 mA). This is as simple as
> it gets but I think this is correct.
>
>
> A better way is to measure the open circuit voltage then apply a
> resistive load to it and adjust the resistance till the voltage is one
> half the open circuit voltage. The load resistance will be equal to the
> source impedance.
>
>
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