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Sad reality of stock vfo

 

Thomas,
That's not optimal behavior for that sort of design. It can do much better than that. I think you should revisit the traditional solutions for such problems and them implement them before moving on- NPO and styrene capacitors might be a start.

John
KC9OJV
 

I can relate to this, and it's why I am switching to a digital VFO. Gluing down L4 was a *huge* improvement for my BITX40. The other thing is to have it in an enclosure that isn't drafty, so that the temperature stays roughly the same if possible. I'm still waiting for the VFO from QRP Labs to get here, but contacts ARE possible with the stock VFO. KC9OJV's suggestion is also a very good one, and were I not going digital, I'd be doing that next.

Ryan?
KC7RYS
Thomas Noel
 

¿ªÔÆÌåÓý

John,

As I said - I knew from others¡¯ experience that the stock design has some significant limitations. I just had no previous personal experience with such builds. I have read of quite a few design mods that offer improvement. The design is simple and easy to mod as Farhan intended. I built the stock design as a starting place for learning.?

I was surprised to see the drift so severe that it is hard to hold frequency during an ¡°over¡±. I have made more than a dozen QSOs in less than two hours of operation. The operator on the other end even commented that I was drifting off frequency.

I would like to continue to experiment with the all-analog radio before moving to the DDS VFO.

Where exactly would you start? Replace the resistors as suggested by Farhan? They were included with my purchase. Which capacitors would you replace to stabilize the VFO, and change just the type, or change any of the values? My tuning range is just where I would want it now.

If anyone can give me a step-by-step attack plan in order of best anticipated return on effort, I would appreciate it a lot.

Thomas W Noel
KF7RSF

On Dec 31, 2016, at 7:28 PM, John Greusel greusel@... [BITX20] <BITX20@...> wrote:


Thomas,
That's not optimal behavior for that sort of design. It can do much better than that. I think you should revisit the traditional solutions for such problems and them implement them before moving on- NPO and styrene capacitors might be a start.

John
KC9OJV
 

I will be frank. This design is a compromise. The VFO should have been on a separate board. I thought about it for a pretty long time. There was no cost difference. There was just one more board to hookup and wire up. In the end and after countless conversations with some co-builders, I decided to include the VCO on-board. I am still convinced that this was a better option than an outboard VFO for easy of use.

Here are some notes from my experimentation.

1. It is NOT about the capacitors. I build a two-tone oscillators using these caps that run at 14.000 and 14.200 MHz directly. They are so solid, I can't believe that it is a free running oscillator.
2. The culprit is heat. The RF amplifiers have significant current, there are three of them. The heat dissipates through the ground plane and heats up the VFO components. This heat would be missing if we used through hole components as those components are off the board.
3. The heat sensitivity is also due to it being a VCO. The varactor tuning couldn't be avoided as getting good quality capacitors is very tough these days.

If you'd still like to retain the analog tuning, do what bill suggests. Build an outboard VFO, shield it well and it will be rock stable.

- f

On Sun, Jan 1, 2017 at 9:44 AM, Thomas Noel tnoel@... [BITX20] <BITX20@...> wrote:
?

John,


As I said - I knew from others¡¯ experience that the stock design has some significant limitations. I just had no previous personal experience with such builds. I have read of quite a few design mods that offer improvement. The design is simple and easy to mod as Farhan intended. I built the stock design as a starting place for learning.?

I was surprised to see the drift so severe that it is hard to hold frequency during an ¡°over¡±. I have made more than a dozen QSOs in less than two hours of operation. The operator on the other end even commented that I was drifting off frequency.

I would like to continue to experiment with the all-analog radio before moving to the DDS VFO.

Where exactly would you start? Replace the resistors as suggested by Farhan? They were included with my purchase. Which capacitors would you replace to stabilize the VFO, and change just the type, or change any of the values? My tuning range is just where I would want it now.

If anyone can give me a step-by-step attack plan in order of best anticipated return on effort, I would appreciate it a lot.

Thomas W Noel
KF7RSF

On Dec 31, 2016, at 7:28 PM, John Greusel greusel@... [BITX20] <BITX20@...> wrote:


Thomas,
That's not optimal behavior for that sort of design. It can do much better than that. I think you should revisit the traditional solutions for such problems and them implement them before moving on- NPO and styrene capacitors might be a start.

John
KC9OJV
 

NP0 capacitors for C94 95 96 WILL tackle the heat problem Farhan!

Happy new year!
Raj

2. The culprit is heat. The RF amplifiers have significant current, there are three of them. The heat dissipates through the ground plane and heats up the VFO components. This heat would be missing if we used through hole components as those components are off the board.
Thomas Noel
 

¿ªÔÆÌåÓý

It is a complete, finished radio for $45!! Of course it is a compromise, and you have always been frank about the design issues.?

I am not complaining. I had fun putting this together, and I¡¯m learning more from this one project than several years of other experiences. Just understanding the costs and compromises of the design decisions, and their effect on the final performance, has been a great lesson, and it is ongoing.

I will continue to explore, first the limits of changes to the analog vfo onboard, and then possible learn to build an outboard vfo.

I also look forward to the opportunity presented by the DDS VFO and learning about the Arduino architecture.

I find it hard to imagine a better way to spend $45 or $59 for an education.

Thomas W Noel
KF7RSF

On Dec 31, 2016, at 10:16 PM, Ashhar Farhan farhanbox@... [BITX20] <BITX20@...> wrote:


I will be frank. This design is a compromise. The VFO should have been on a separate board. I thought about it for a pretty long time. There was no cost difference. There was just one more board to hookup and wire up. In the end and after countless conversations with some co-builders, I decided to include the VCO on-board. I am still convinced that this was a better option than an outboard VFO for easy of use.

Here are some notes from my experimentation.

1. It is NOT about the capacitors. I build a two-tone oscillators using these caps that run at 14.000 and 14.200 MHz directly. They are so solid, I can't believe that it is a free running oscillator.
2. The culprit is heat. The RF amplifiers have significant current, there are three of them. The heat dissipates through the ground plane and heats up the VFO components. This heat would be missing if we used through hole components as those components are off the board.
3. The heat sensitivity is also due to it being a VCO. The varactor tuning couldn't be avoided as getting good quality capacitors is very tough these days.

If you'd still like to retain the analog tuning, do what bill suggests. Build an outboard VFO, shield it well and it will be rock stable.

- f

On Sun, Jan 1, 2017 at 9:44 AM, Thomas Noel?tnoel@...?[BITX20]?<BITX20@...>?wrote:
?

John,


As I said - I knew from others¡¯ experience that the stock design has some significant limitations. I just had no previous personal experience with such builds. I have read of quite a few design mods that offer improvement. The design is simple and easy to mod as Farhan intended. I built the stock design as a starting place for learning.?

I was surprised to see the drift so severe that it is hard to hold frequency during an ¡°over¡±. I have made more than a dozen QSOs in less than two hours of operation. The operator on the other end even commented that I was drifting off frequency.

I would like to continue to experiment with the all-analog radio before moving to the DDS VFO.

Where exactly would you start? Replace the resistors as suggested by Farhan? They were included with my purchase. Which capacitors would you replace to stabilize the VFO, and change just the type, or change any of the values? My tuning range is just where I would want it now.

If anyone can give me a step-by-step attack plan in order of best anticipated return on effort, I would appreciate it a lot.

Thomas W Noel
KF7RSF

On Dec 31, 2016, at 7:28 PM, John Greusel?greusel@...?[BITX20] <BITX20@...> wrote:


Thomas,
That's not optimal behavior for that sort of design. It can do much better than that. I think you should revisit the traditional solutions for such problems and them implement them before moving on- NPO and styrene capacitors might be a start.

John
KC9OJV
 

raj,

the varactors have a bad temperature coefficient. they contribute too.

- f

On Sun, Jan 1, 2017 at 11:52 AM, Raj vu2zap@... [BITX20] <BITX20@...> wrote:
?

NP0 capacitors for C94 95 96 WILL tackle the heat problem Farhan!

Happy new year!
Raj

>2. The culprit is heat. The RF amplifiers have significant current, there are three of them. The heat dissipates through the ground plane and heats up the VFO components. This heat would be missing if we used through hole components as those components are off the board.
Rahul Srivastava
 

Hi ,

Can try putting a couple of 4148 on either side of tuning pot depending upon the direction to provide some compensation for thermal drift.

Rahul VU3WJM


On Sun, Jan 1, 2017 at 1:17 PM, Ashhar Farhan farhanbox@... [BITX20]
wrote:
?


raj,

the varactors have a bad temperature coefficient. they contribute too.

- f

On Sun, Jan 1, 2017 at 11:52 AM, Raj vu2zap@... [BITX20] <BITX20@...> wrote:
?
NP0 capacitors for C94 95 96 WILL tackle the heat problem Farhan!

Happy new year!
Raj

>2. The culprit is heat. The RF amplifiers have significant current, there are three of them. The heat dissipates through the ground plane and heats up the VFO components. This heat would be missing if we used through hole components as those components are off the board.
 

I'm sorry Thomas,

In my New Years festive mood I misread your first post and thought you were talking about an earlier design- not the new Bitx40 smd version. Farhan is, of course, the expert and he knows the ins and out completely. Heat seems to be the usual suspect and perhaps NPO caps might be some benefit but ultimately a digital VFO may be the answer you're looking for.
Unless! you want to take a detour and take a huff and puff circuit for a spin. :)

John
KC9OJV


From: "Thomas Noel tnoel@... [BITX20]"
To: BITX20@...
Sent: Saturday, December 31, 2016 10:14 PM
Subject: Re: [BITX20] Sad reality of stock vfo

?
John,

As I said - I knew from others¡¯ experience that the stock design has some significant limitations. I just had no previous personal experience with such builds. I have read of quite a few design mods that offer improvement. The design is simple and easy to mod as Farhan intended. I built the stock design as a starting place for learning.?

I was surprised to see the drift so severe that it is hard to hold frequency during an ¡°over¡±. I have made more than a dozen QSOs in less than two hours of operation. The operator on the other end even commented that I was drifting off frequency.

I would like to continue to experiment with the all-analog radio before moving to the DDS VFO.

Where exactly would you start? Replace the resistors as suggested by Farhan? They were included with my purchase. Which capacitors would you replace to stabilize the VFO, and change just the type, or change any of the values? My tuning range is just where I would want it now.

If anyone can give me a step-by-step attack plan in order of best anticipated return on effort, I would appreciate it a lot.

Thomas W Noel
KF7RSF

On Dec 31, 2016, at 7:28 PM, John Greusel greusel@... [BITX20] <BITX20@...> wrote:


Thomas,
That's not optimal behavior for that sort of design. It can do much better than that. I think you should revisit the traditional solutions for such problems and them implement them before moving on- NPO and styrene capacitors might be a start.

John
KC9OJV

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