Friday, 21 February 2014

GPS disciplined clock/oscillator

GPS disciplined clock/oscillator

In the electronic wilderness before I re-discovered amateur radio, I have had an interest in mapping from Boy Scouts, and GPS, from boating and 4WD (we belonged to the Mitsubishi 4WD club). I have had GPS devices ever since President Clinton opened GPS for civilian use.

I had an ALDI GPS mobile phone tracking system going, but never used. Pretty neat for $80. While Telstra don't advertise it, there is a $10 per month post-paid mobile access that suits such devices. All use is on top of the monthly charge. The tracking device reports its position when requested by SMS. Expensive if running all the time, but cheap if only used when needed, like locating a vehicle in congested traffic to find an alternative route.

Enough background. Through my local radio club, I heard about GPS disciplined 10 MHz oscillators, so I just had to have a look. They are easy to find on eBay, but not so easy to find information, documentation or software. Given I have a lot of time to wander the ether, I managed to find most of what I needed.

I duly order the GPSDC (Trimble Nortel GPSTM NTGS50AA $120), antenna (Nokia 470290a-101 Timing Reference Antenna $20) and power supply (-48V $25) from China where they disassemble old mobile phone equipment. The fellow running the store was very helpful. With everything arrived over a couple of weeks, I had a last minute panic on the coax connectors; they were odd and I couldn't find what type they were. The man from China said they were a variation of  SMB. So with overnight service from element14 (ne-Farnells), I had SMB to SMA adaptors and on familiar territory.

I wasn't planning to get it going last night, but "Big Bang Theory" was still in its St Valentines recess, so I started connecting it all up. Fortunately the -48 V is floating and the board is grounded. CDMA mobile gear uses either 24 V or -48 V; curious? The GPSDC connects to a PC via a serial port. I have a long-standing (35 years) dislike of serial ports! Running Windows 8 on a laptop with a serial to USB converter, trying to run programs written for Windows 95 or NT is courting trouble. However it eventually worked long enough to detect one satellite, with the antenna indoors, before crashing, verifying that the thing worked. Using "Lady Heather" I was going. Putting the antenna outside and leaving it run overnight, I woke up to a green "Locked" LED. The scope showed the 10 MHz I was after.

Lady Heather, bit squished but can be fixed

 
 
That beautiful green light that says locked. Orange one I think is just on/off.


 
 
Elaborate mount for GPS antenna. The ladder did fall over in the wind and broke an expensive coax adaptor. 

 
 
The board with a CRO showing the locked 10 MHz output, 2 V peak to peak.


Saturday, 8 February 2014

New TRX- IC-7100, a look inside

New TRX- IC-7100, a look inside

I have just purchased a new IC-7100 to replace the FT-857 and FT-897 all-band, all-mode TRX I had. The Yaesu TRX were good radios, but dated designs.

The IC-7100 on the other hand is a very modern design and radical form with a separate touch screen controller. The IC7100 is a similar but different design to other modern Icom TRX such as the IC-7410 and IC-9100. All are software-defined radios, with all modulation and demodulation done in software/firmware in the DSP and codec chips. All other signal processing is also done in software.

Analogue devices are used for amplifiers and mixers. Band, low-pass and roofing filtering is done with discrete components, but all signal filtering is done by the DSP.

I will say more of the design, but for the moment I have some photos of the main unit's PCBs. The mechanical design is simple.

The top board is the frontend for both Rx and Tx with most components of one side of the board mainly because of the size of the filter components.

 
 
The TX transistors do all bands, 1.8 - 470 MHz. The transistors are bolted to a serious fan-cooled heat sink, seen from underneath, with the small signal board folded to the side..
 
 

The small signal board is connected by one coax cable for both RX and TX RF, plus two flat cables for all other connections.


On the small signal board there are components on both sides of the PCB. The top side has all the DSP chips. There are three computers running the TRX.


The single coax cable carries both RX and TX between the two boards. This connector is of special interest for tapping point for an SDR, after the all the TRX frontend and filters. An opto-coupler will be necessary to turn off the tap during TX. This is the case for some Yaesu TRX first IF taps and hopefully will be straightforward.


So, a compact, modern designed TRX, easy to access for servicing and the like.