[minima] Re: Experimenting with the crystal filter
- From: Ashhar Farhan <farhanbox@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Fri, 24 Jan 2014 16:10:19 +0530
adrian, it would be around 220 pf or less, i would suspect. the greater
challenge would be to handle the low impedance. we'd expect that we will
need something like 10 ohms or so.
it might be lossy. somebody can place 220pfs and give it a scan. i'm going
to be a bit qrl until this weekend to heat up the iron.
- f
On Fri, Jan 24, 2014 at 3:22 PM, Adrian Preda <ady.preda@xxxxxxxxx> wrote:
> Hi,
>
> What caps values can be used for a CW filter, like 300-400Hz?
>
> Adrian
> YO4HHP
>
> ------------------------------
> *From:* Mark G0MGX <mark.g0mgx@xxxxxxxxx>
> *To:* minima@xxxxxxxxxxxxx
> *Sent:* Friday, January 24, 2014 11:42 AM
>
> *Subject:* [minima] Re: Experimenting with the crystal filter
>
> Interesting reading list!
>
> Just shows you - I suspect the filter you have now is just great.
>
> M
>
>
> On 24/01/2014 09:34, Thomas Sarlandie wrote:
>
> Mark,
>
> No, I had not. I have a bag coming. I will experiment with the circuit
> and method (inspired by g3uur) shown in the ARRL handbook. I also found
> some interesting doc online:
> http://pages.suddenlink.net/wa5bdu/crystal_slide_show.pdf and
> http://openqrp.org/?p=418. I am traveling this weekend but that should
> give me some good theory to be ready when I come back.
>
> I will probably move on to another stage before I get back to it though.
>
> thomas
>
>
> On Fri, Jan 24, 2014 at 1:28 AM, Mark G0MGX <mark.g0mgx@xxxxxxxxx> wrote:
>
> Hi Thomas
>
> That looks much better!
>
> I don't remember if you said you had matched the crystals or not?
>
> Mark
> G0MGX
>
>
> On 24/01/2014 09:24, Thomas Sarlandie wrote:
>
> Hey,
>
> It occurred to me this morning that I had measured the output
> peak-to-peak voltage of the DDS in open circuit and calculated the power
> like if it was 50 ohm terminated...
>
> Sure enough with a 50 ohm termination (after the L-match), the output
> power of the circuit is -5.9 dBm and the characteristic of my filter much
> more normal:
>
> 2 dB bandwidth: 4.50 khz
> Insertion loss: -5 dB
>
>
> https://docs.google.com/spreadsheet/ccc?key=0As9CZnZ-A5a2dEZaM1dlWVdVX3pXaUlJMXd0cHNscHc#gid=1
>
> lesson learnt!
>
> thomas
>
>
> On Thu, Jan 23, 2014 at 3:56 AM, Mark G0MGX <mark.g0mgx@xxxxxxxxx> wrote:
>
> Hi Gang
>
> I just pulled 8 crystals out of the bag and built another crystal filter
> without attempting to match them - just out of curiosity.
>
> The sweep is attached, the -2dB bandwidth is 3.5KHz and the insertion loss
> 4dB (the tracking generator is at -20dBm).
>
> The sides of the filter don't look quite as steep but it still looks
> pretty good without any matching - maybe I got lucky? Certainly can't
> explain why you are seeing a 10dB insertion loss - that's very high.
>
> Mark
> G0MGX
>
>
> On 23/01/2014 10:42, Thomas Sarlandie wrote:
>
> 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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