EMRFD Message Archive 1554
Message Date From Subject 1554 2008-04-14 07:11:22 Ashhar Farhan DTC losses
front-end. I am puzzled by the high loss of 16db. I am not really sure
if I am measuring it correctly. so, here is the puzzle.
1. I strapped together a simple crystal oscillator like the one shown here:
http://phonestack.com/farhan/testosc.gif
2. it is followed by a pi attenuator with 330 ohm legs and a 22 ohm
across them. this is to create an approximately 50 ohms output.
3. I designed a dtc with 1.8uH inductors, appox 60 pf tuning
capacitors across them, 2.2 pf coupling capacitor between them and a
10pf in series with 100pf on either sides to 'tap down' the impedence.
4. I terminate the filter in a 50 ohms load and it is driven by the
output of the pi attenuator.
5. at the input of the filter, across the 100pf, i can measure more
than 200 mV input. However, at the output, after tuning the filter, i
don't get more than 50 mV output.
The DTC peaks smoothly. At first, I suspected the Q of the coils. I
have since tried replacing the original inductors (polyester core
toroids) with air core coils wound with 36 gauge wire. Both give
almost the same readings.
Is there anything that I am doing wrong?
- farhan1555 2008-04-14 07:11:25 Ashhar Farhan DTC losses
front-end. I am puzzled by the high loss of 16db. I am not really sure
if I am measuring it correctly. so, here is the puzzle -
1. I strapped together a simple crystal oscillator like the one shown here:
http://phonestack.com/farhan/testosc.gif
2. it is followed by a pi attenuator with 330 ohm legs and a 22 ohm
across them. this is to create an approximately 50 ohms output.
3. I designed a dtc with 1.8uH inductors, appox 60 pf tuning
capacitors across them, 2.2 pf coupling capacitor between them and a
10pf in series with 100pf on either sides to 'tap down' the impedence.
4. I terminate the filter in a 50 ohms load and it is driven by the
output of the pi attenuator.
5. at the input of the filter, across the 100pf, i can measure more
than 200 mV input. However, at the output, after tuning the filter, i
don't get more than 50 mV output.
I am using my tektronics scope with a caliberated 10db probe to make
the measurements at both ends, parallel to the input.
The DTC peaks smoothly. At first, I suspected the Q of the coils. I
have since tried replacing the original inductors (polyester core
toroids) with air core coils wound with 36 gauge wire. Both give
almost the same readings.
Am I doing anything wrong?
- farhan1556 2008-04-14 16:06:10 Johan H. Bodin Re: DTC losses
you may wish to try the free filter design program from AADE:
http://www.aade.com/
It can be used to either design a filter from scratch or to enter an existing
design and play with the values to see "what if...".
73
Johan SM6LKM1557 2008-04-14 16:06:21 wimmie262000 Re: DTC losses 1558 2008-04-15 06:31:52 Ashhar Farhan Re: DTC losses
to hundreds of bitx out there!! i had quickly strapped this together
without any design thought while building my original bitx and it
stayed that way.
this week, i tried measuring the filter and found this problem. let me
give a heads up to the bitx group as well...
- farhan