[minima] Re: Experimenting with the crystal filter
- From: Vladimir Ljubojevic <vljubojevic@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 17 Feb 2014 00:23:11 +0100
Hello,
Has anyone so far built or designed a narrow cw filter for their Minima?
Fric
YU1WC
On Fri, Jan 24, 2014 at 11:40 AM, Ashhar Farhan <farhanbox@xxxxxxxxx> wrote:
> 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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>
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