Frequency Synthesizer Applications as an Oscillator for a HF Receiver?

W8VK · 2011-07-12 15:38

Hello!
I am using the PropRPM V1.0 Board as a Frequency Synthesizer! The Frequency is varible from 3.000.000 Mhz to 3.125.000 Mhz. This Frequency change is done via the CommPort thru a Visual Basic Express 2010 Program very nicely!
I am trying to use this output from the prop board to replace a receiver varible oscillator but have run into the following problems!
1. I'm assuming this is an RF signal that can be amplified to provide a bigger rf output to be applied to a mixer ic to come out at a desired receive frequency range. The present signal level appears to be too weak.
2. What sort of a signal amplifier has to be used to amplify this small signal to a proper level. I've tried a simple 2N4401 Transistor circuit and a Sa612 ic with no results! (Any suggestions and circuit examples would help)
3. P0 Pin is the output pin off my prop board. As i move a wire up and down this row, the output gets less and less as i near the the edge of the board using the P0 row while monitoring the signal strength on a receiver! Anyone else notice this occuring too?
All help and comments are appreciated and welcome. Thanks for reading this post!

Rich W8VK

Toby Seckshund · 2011-07-12 16:16

I have thought about using the Prop for this too. The usual, and valild reply is that there is a huge amount of jitter (phase noise) from the way that the CNT runs in this mode. Yes it looks fine and dandy on the 1 sec sampled frequency counter, but it bends all over the place. Lots of things have been surgested to calm this down, such as PLL or even just a LC (for narrow BW).

I am thinking about a way of generating the likes of "90 - 120MHz" via an analogue PLL and bringing it back down to baseband as do the SDRs

localroger · 2011-07-12 17:01

I would think that for most purposes, especially for a receiver, even with the jitter the Propeller counters would work fine. Since you're not broadcasting you can decide for yourself if there's too much noise.

1. The output of a Propeller pin is a pretty robust signal, 3.3 volts peak to peak at 30 mA. However, it might be at a lower impedance than the circuit you're feeding. You might need a tuned transformer (which would help with the jitter too) to boost the voltage if you're injecting into a stage that expects say 12V p-p.

2. See #1. It's most likely not amplification you need, but impedance transform to a higher voltage at lower current. An amplifier that boosts the current doesn't help if you need voltage.

3. The pin pads on a PropRPM don't go down the whole row, most of the board is just bare pads with no interconnections at all. The further you get from the P0 output pad the weaker the signal will be.

Mark_T · 2011-07-12 17:20

For an RF local oscillator its important to have good spectral purity - using the counter modules isn't going to give you this - the result is utterly unusable for transmit, and very poor selectivity on receive - each frequency present in the local oscillator will mix a signal frequency down to the IF. Basically you can't get away with it.

However there are chips available that do it properly, a popular one is the AD9851 which runs at 180MHz and can generate DC upto 50MHz via direct digital synthesis (DDS) using a 10 bit high speed DAC. Google for this and you'll find a lot of radio amateur projects using it. There are slower chips that are cheaper if 3MHz is all you need - in fact any high speed DAC that goes up to 10Msps coupled with a suitable fast ROM is going to give you DDS (you need a sine table). There are also some high end DDS chips with 14 bit DACs and some that clock at 1GHz or so - expensive!

All of these devices need a grounding in RF circuit design to use BTW and careful attention to detail to limit the amount of digital noise getting into the output. There are still issues of spectral purity - 10bits is only 60dB of signal to noise at best - OK for mediocre receivers and low power transmitters.

The alternative of putting the Prop output through a PLL to filter out the phase noise will work too (but to filter the phase noise I think you need a _very_ narrow LPF in the PLL - this will prevent use of digital PM or FM I suspect).

By starting with a pure enough signal DDS allows phase and frequency modulation in the digital domain which is very cool.

[edit: a useful reference for RF design
http://www.arrl.org/arrl-handbook-2011 ]

Phil Pilgrim (PhiPi) · 2011-07-12 18:35

localroger wrote:

You might need a tuned transformer (which would help with the jitter too)...

This, unfortunately, does not work. The jitter produces spurs (i.e. sidebands) close enough to the fundamental frequency that they can't be filtered out with a simple LC filter.

My best success producing clean RF with the Propeller has been to use DUTY mode output to control a varactor in an external oscillator and to frequency- and phase-lock that oscillator with software:

http://forums.parallax.com/showthread.php?123791

Lately, I've begun experiments with the oscillator in the SA612 in hopes of building a single-band ham receiver with the Propeller doing the frequency selection, demodulation, filtering, and volume control.

-Phil

IanM · 2011-07-12 21:27

Rich, you could try my DDS. It has an equivalent clock frequency of 20MHz and will work fine generating 3MHz. It's still experimental and uses 7 synchronised cogs. It generates quadrature signals through two 8 bit R2R ladder networks. Please see http://forums.parallax.com/showthread.php?121470-A-Sine-Wave-DDS-in-PASM-Comments&p=989118&viewfull=1#post989118

jmg · 2011-07-12 21:41

Why not use a device like SI5351A-A-GT ?

8Khz to 160MHz, in MSOP10, and three outputs, In stock at Digikey and just $1.34 1+

Phil Pilgrim (PhiPi) · 2011-07-12 22:45

The Si5351 is not tunable. You have to select the frequencies you want when you order it.

-Phil

jmg · 2011-07-13 03:28

Phil Pilgrim (PhiPi) wrote: »

The Si5351 is not tunable. You have to select the frequencies you want when you order it.
-Phil

? Not according to the data sheet. It has OTP memory that loads into RAM, and that you can pre-program to give default values, but the RAM is written via i2c, and the data says this :

During a power cycle the contents of the NVM are copied into random access memory (RAM), which sets the device configuration that will be used during normal operation. Any changes to the device configuration after power-up are made by reading and writing to registers in the RAM space through the I2C interface.

Their ClockBuilder software creates register-array copies, if you want a simple table (maths free) load and go.
Some of the other SiLabs parts have no i2c bus, and are OTP only.

Phil Pilgrim (PhiPi) · 2011-07-13 08:18

jmg,

You might be right about that. The datasheet does not describe any I2C-accessible registers related to frequency, and that's what I based my comment on. However, there is a large block of registers that it does not describe but refers the user to their ClockBuilder software to define the correct values. If these registers set the divider values, it's understandable that such a program would be helpful. Hopefully, there would still be a way to sweep through a reasonable frequency range gracefully without resorting to a complex algorithm or multiple register settings for each incremental change.

-Phil

localroger · 2011-07-13 08:28

That's good info, Phil. Experimenting with the PLL's for radio was one of those things on my long future to-do list. Looks like I can save myself some trouble.

Michael OBannon · 2011-07-13 09:50

The Si570 is a component you should consider. It has been used successfully in a number of SDR designs and is definitely tunable via I2C. A number of microprocessors (PIC, ATMEL, PICAXE, TI, etc) have been used as controllers thus far. The Propeller could easily serve as a controller. Output is a square wave which is good for driving quadrature switching mixers and passive diode mixers.

Here are couple of links from the ham radio world with useful information:

http://homepages.wightcable.net/~g4zfq/Si570.htm
http://mbed.org/cookbook/Si570

I have a Si570-based VFO here on the bench that works well for experimenting with RF designs.

Michael KD4SGN

jmg · 2011-07-14 14:34

Michael OBannon wrote: »

The Si570 is a component you should consider. It has been used successfully in a number of SDR designs and is definitely tunable via I2C.

The Si570 specs are above the Si5351A, and it includes a crystal, and specs a 'some GHz VCO', and it is of course also quite a bit more expensive (but still affordable, given the specs).

So for premium use the Si570 is a better choice, but it would be good to know just how much worse the Si5351A is, in Radio type uses ?

Mark_T · 2011-07-14 17:57

The Si570 only goes down to 10MHz so some external division is needed - however that can be used for generating quadrature of course

W8VK · 2011-07-17 08:29

localroger wrote: »

I would think that for most purposes, especially for a receiver, even with the jitter the Propeller counters would work fine. Since you're not broadcasting you can decide for yourself if there's too much noise.

1. The output of a Propeller pin is a pretty robust signal, 3.3 volts peak to peak at 30 mA. However, it might be at a lower impedance than the circuit you're feeding. You might need a tuned transformer (which would help with the jitter too) to boost the voltage if you're injecting into a stage that expects say 12V p-p.

2. See #1. It's most likely not amplification you need, but impedance transform to a higher voltage at lower current. An amplifier that boosts the current doesn't help if you need voltage.

3. The pin pads on a PropRPM don't go down the whole row, most of the board is just bare pads with no interconnections at all. The further you get from the P0 output pad the weaker the signal will be.

I didn't realize their were no pad interconnections down the rows! Ha! Good Information! Thanks!

Rich W8VK

W8VK · 2011-07-17 08:42

Yea I'd don't understand things yet either! I listen to this Frequency Synthesizer on a communications receiver on 2 to 30 Mhz Ham and Short Wave Frequencies and I don't detect ANY Drift at all! Seems stable enough to use as a Mixer Oscillator to an IF, and as a Varible Oscillator. Maybe it is an impedance mismatch as you say to the Sa612/2N4401. Thanks for your reply!

Rich W8VK

Phil Pilgrim (PhiPi) · 2011-07-17 08:51

Tune your receiver a few tens or hundreds of KHz on either side, and you'll hear it again; and so on, up and down the band. The problem is not lack of stability; it's lack of purity. As a result you could be mixing in image signals from those spurious local oscillator sidebands.

-Phil

W8VK · 2011-07-17 08:52

Yes I think I do notice the noise factor in the receiver! Maybe a LPF can be used from the generating pin that would limit it in the Target Frequency range for each Ham and Short Wave Band to be used! This might even match the impedance as the one posting was suggesting. I guess a Transistor stage could then be a Voltage amplifier then if needed? Don't really know if i need much amplification as really needing a buffering/isolation action!
Thanks for the posting!

Rich W8VK

W8VK · 2011-07-17 08:56

Yes Phil!
I'm noticing some other frequencies generated about every 17 Khz in the 3 Mhz i have been testing at! Thanks!

Rich W8VK

W8VK · 2011-07-17 09:14

Hi Mark_T!
I don't have the 2011 Handbook and would have to buy one to see this info! Seems like this DDS Stuff takes lots of parts and needs more work for computer control and a Frequency Display is Quite involved!

I did a BS2 Project 2 years ago using a varible resistor in place of a ten turn pot! It would tune fine, But heat in the pot caused too much drift! Plus the BS2 was very slow controlling thru visual basic and the BS2 let me do basic which is great to program in!

I wish some of these guys would write up the DDS projects in detail for people with all the design basics and very easy to grasp to learn the techniques involved. I haven't seen one simple enough to get excited enough to play! Got turned off and lost in the PIC associated along with the project. And the Programming of the Pic to run the processor! Thanks for your reply!

Rich W8VK

Leon · 2011-07-17 10:18

Here is a DDS project of mine from some years ago:

http://webspace.webring.com/people/jl/leon_heller/dds.html

It used the AD9850, the first DDS chip produced by ADI.

N3JQD · 2011-07-17 16:25

This is a great thread. I read it with great enthusiasm and I am tryiong to use my BS2 for anything ham (amateur) radio related. Kudos. I'll monitor this thread.

W8VK · 2011-07-17 21:59

Hello N3JQD!
Thanks for your posting! I did a BS2 project as a Directory that would turn on off power to the receiver PS, Control a Digital resistor I used to control a modified Small Wonder Labs SW40 VFO circuit! It worked but the BS2 was so slow as compaired to the PropRPM I have. Another problem I had was the digital pot got warm and would drift frequency! Unless there is an Audio pot with some wattage as a Digital pot, the idea isn't that great!
I had hoped the prop freq synthesizer would offer more but can't isolate the signal for one or amplify it to drive a simple SA612 mixer for a receiver.
Although i Wrote about 40 Visual Basic 2010 programs for 13 short wave bands, and 10 hf ham bands, and the us cb band. Sounds great on a receiver and changes frequency very nicely! But can't get by the other issues in this posting! Would be so much nicer of a way to play with programmable mixers and vfos as compaired to this dds stuff! I havent even seen any simple in depth articles written for dummies or beginners to really grasp the basics! Thanks for the post and God save us all! Ha!

Rich W8VH

W8VK · 2011-07-19 17:34

Hi all!
This will probably be the last posting for this project!

1. My Visual Basic 2010 programs sure controlled the overall range of the proprpm board nicely. In fact i had all ham bands for cw mode 160-10 meters. (See attached picture)

2. Seems to be a purity issue of the signal. (Looks like Phil Pilgrim will whip that issue)

3. The spin programs are attached for those interested! (Makes a cheap signal generator though)

4 In summary, I hope someone will come out with a vfo we can all play with on the Prop PCBs!

Thanks to all for their interest in my project!
Rich W8VK