« Feeding a DDS with a GPS disciplined oscillator – part 2 | Main | Improved remote station audio link »

Remote Operation of a Flex 5000

I've made slow progress over the last few weeks on my Gen 2 WSPR system due to the (pleasant!) distraction of integrating a new Flex-Radio 5000 into my remote station.

I thought long and hard about the choices for a new primary transceiver to displace the Kenwood TS-B2000 I've used for the last 6 years. It came down to a choice between the Elecraft K3 and the Flex Radio 5000. Both are excellent radios with outstanding receiver performance. I opted for the Flex 5000 because of the attraction of being able to extend the radio's functionality through software and PC horsepower. The choice was easier for me as I've operated remote for so many years I don't miss the tuning knob and physical buttons of a radio control panel!

When I first setup the Kenwood TS-B2000, I worked very hard to avoid having a computer at the remote location (in my case, about 500 feet from the house – close to the antennas). I opted to control the Kenwood via an Ethernet connected terminal server (4 RS-232 ports capable of operating over Ethernet at up to 115.2 Kbps) and transfer bi-directional audio over an extension of the small PBX in the house. An auto-answer telephone coupler and a modified MFJ auto-patch interface the phone to the remote radio and my desktop computer.

I use three ports on the terminal server:

  • CAT control of the TS-B2000 at 57,600 Bps
  • PTT control & audio interface via a RigBlaster Nomic
  • Serial connected relay controller for switching

I modified the MFJ auto-patch to operate in reverse as audio coming from the telephone line goes to the audio input on the PC sound card; audio from the microphone needs to drive the telephone line. I added a small audio amplifier (LM386) to provide more drive between the microphone and the line. A 1K resistor over the connector to the handset port simulates "off-hook" to keep the telephone line open. The PBX port auto-dials the remote auto coupler when the phone patch goes "off-hook".

Initially I used an automatic hang-up detector at the remote end that detected the line disconnect tone from the PBX. This was fine for voice or operation but would sometimes false detect when operating CW or digital modes and hang up in mid-stream! I solved this problem by using the relay controller to drive the hang up control line on the auto coupler.

A side-note – my first idea for the audio transport was to use Voice over IP to support bi-directional audio. I tested this out but found there was too much delay introduced both on receive and transmit. Cracking a pile-up was almost impossible. Since then, a lot of progress has been made on codec performance and PC horse power – the Skype SILK codec works pretty well – still not as good as a analog solution but acceptable for rag chewing and casual DX operation.

I ordered the Flex and set about building a simple audio switching unit so that I could select either the Flex 5000 or the Kenwood to source and sink audio via the auto-coupler. A relay on the serial relay controller determines the audio source – Flex or Kenwood.

The Flex requires a PC at the remote end to run PowerSDR – the software "guts" of the radio that provides the DSP capabilities. Since I was going to control the remote PC using UltraVNC (a great open source solution for remote desktop control), run PowerSDR plus a raft of digital mode software (Spectrum Lab, WSPR, MixW…), I wanted to make sure I had plenty of PC horsepower to spare. I purchased an HP Pavilion desktop machine with an Intel Quad Core (2.33 GHz/core) which unfortunately came with Vista X64.

Vista doesn't provide anything I need and between the 64 bit OS and driver signing issues, is frankly a pain. It works fine and its stable but the "enhanced security" with its endless "do you really want to do this" dialog boxes is a hassle. This has NOTHING to do with Flex – indeed, once configured Vista X64 runs the Flex software perfectly.

At some point in the next couple of weeks, I'm going to add a Windows XP dual boot capability to the system and run XP as the default system.

With UltraVNC, PowerSDR, DXWin logbook, MixW or WSPR running, typical CPU utilization is less than 15%!!! I have had NO issues at all with the PowerSDR software disconnecting from the Flex Firewire driver due to excessive latency. Screen updates and mouse control is very snappy – UltraVNC runs about 20 MBps over the Ethernet (100BaseT in my case) and causes very little load on either my desktop or the remote machine running the Flex.

The Flex 5000 is a spectacular performer on all modes. Having brick wall filters with full control over bandwidth and center frequency makes operation on all modes a piece of cake.




Had a quick Q regarding your WSPR v1 design:

Everything else I see regarding WSPR shows sending the 4 FSK audio tones into an SSB exciter.

Your design is (or seems to me to be) shifting the carrier by the same frequency offset as the tones.

If I want to build a WSPR only transmitter, yours seems like the simpler approach.

Here I'm sure I'm showing my lack of understanding of SSB modulation theory, but do both techniques end up sending out the same signal? Or if not exactly the same it must be close enough to make the receiver happy (given that it works)?



An SSB transmitter generates a signal by feeding audio into a balanced modulator - the modulator is essentially a bridge that takes audio in on one leg and the initial RF carrier frequency on the other.

When the modulator is balanced and there is no audio, there is no RF out of the modulator.

Apply audio and you get mixing between the audio and the RF carrier - this generates a DOUBLE sideband signal - RF + and - the modulating audio frequency.

The next stage of an SSB transmitter is typically a crystal filter that removes one of the sidebands - the resulting SSB signal is then mixed to the required operating frequency and amplified.

So, if you take an SSB transmitter and feed in a single audio tone, you get RF generated at the center carrier frequency (the frequency shown on the transmitter VFO) PLUS or MINUS the audio frequency depending on the sideband selection (PLUS for upper sideband).

So, a single audio tone fed into an SSB transmitter generates a carrier of constant frequency - this looks exactly like a crystal oscillator or DDS synthesizer output.

If you take 30M as an example, the WSPR center carrier frequency for SSB operation is 10.1387 MHz. The WSPR software generates a tone between 1400 and 1600 Hz that when transmitted appears at a carrier frequency of between 10.1401 and 10.1403 MHz. The actual frequency depends on the configuration of the WSPR software.

The software version of WSPR transmits one of four audio tones that vary from the configured frequency by multiples of 1.47 Hz.

You can generate exactly the same effect by shifting a crystal using a varicap diode - that's the method used in my WSPR V1 transmitter - the varicap in my case is an LED diode - inexpensive LEDs demonstrate variable capacitance depending on the level of reverse bias but over a narrower range of capacitance than a varicap diode.

Since the required frequency shift is very small, the LED provides more than enough capacity to FSK the crystal by the required levels.

Hope this helps clear up any confusion!

Best 73's
Stu K6TU

Perfect -- thanks!

The comments to this entry are closed.

My Photo


Intense Brit, lived in Silicon Valley since 1984. Avid pilot, like digital photography, ham radio and a bunch of other stuff. Official Geek.

© 2006 - 2014 Stu Phillips

All Rights Reserved.