The most important board the repeater group developed is the "Logic Interface". This unit is connected between the radio system and the RC210 Controller and offers full signal conditioning/isolation of control lines as well as impedance matching of the audio stages.
The interface also detects and triggers the repeater logic on reception of a 1750Hz tone access. The 1750Hz detector also feeds both a self tune notch filter and a 80dB attenuater. Both systems notch out the 1750Hz component from the through audio.
** NOTE ** As per the announced in our 2013 newsletter, due to a fault the 1750Hz tone detection as been turned off on GB3BS in favour of the CTCSS Tone detection, as all new repeaters are to be CTCSS only so the drive to investigate and resolve the tone detection was not persued.
600R/600R Audio Match and Isolation
1750Hz Tone Detection (Now disabled)
2 Method of 1750Hz Notch (Now disabled)
Approx -80dB Notch attenuation at 1750Hz
Electrical buffering of control lines
"Drop Dead" P.T.T. Protection
CTCSS and SPEECH detection indication
The custom logic contol interface
GPS Derived Real Time Clock
Although the RC210 has an on-board Real time Clock (RTC) over a period of time this clock does drift. This is due to CPU overhead and other environmental influences. The repeater does have the capability to semi correct this drift, this correction is a pre-defined value on a pre-defined time.
This can lead the correction process causing the RTC to be adjusted more than it needs as it cannot be guaranteed how much the RTC has drifted by.
GB3BS uses the RTC to define our automatic beacons which, as defined by the BR68 have to be transmitted at a period no longer than 15 minutes.
To maintain accuracy The Bristol 70cms Repeater Group investigated using an "off air" time source, we researched a number of time solutions before settling on GPS TIME.
GPS Time has a mean accuracy of around 28ns RMS per 19 years if uncorrected. However the Department of defence (U.S.) correct the 3 Atomic clocks that each spacecraft has at least once in any given 24hr (earth time) therefore over a period of a standard earth day each spacecraft is re-synchronised! This maintains the accuracy of the time received.
It is worth noting for the purists out there, that GPS time is currently 19 seconds adrift of TAI Time (International Atomic time). This is due to GPS time not being based on fixed star to fixed star time unlike how time on earth is measured, because of this fact GPS Time is not correct using leap seconds as TAI (Earth time) is. Thankfully most GPS Receivers receive and process the current offset information that is transmitted along with the Satellite almanac thus the received GPS time is received from Satellite, corrected by the Almanac offset and at this point is displayed. The resulting time is as accurate if not more accurate than the time standard that used to be broadcast from Rugby.
Once a GPS time source was decided Mat then had to develop a system that could receive and process this GPS TIME then update the RC210. The result is a small unit that is separate (at the time) to the Repeater Rack, this unit has a commercially available GPS Receiver and custom designed electronic interfacing and a PIC Micro controller running "In House" code. Since Mats original design in 2008 the GPS clock has had 4 complete hardware re-designs, and a number of software re-writes. Mat has taken the original strip-board MK 1 design and turned it commercial product now uitilising a 32bit Arm processor and the modern ublox based GPS chipset. A number of new software features were also added allowing the clock to work anywhere in the world in any timezone as well as selectable auto daylight saving (DLS). More features are in the pipeline. For more information take a look at his website: https://technoloya.co.uk
Over a 24hrs period the RC210 has its RTC updated48 times, this is more than the RC210 would normally need, but it was decided that putting this much effort into getting a accurate time source then we should make sure the RC210 is as accurate as possible. The RTC date is also updated at the same frequency. Although we do not heavily use the date feature, there are groups else where in the world that do use the RC210's date feature almost like a diary allowing them to create trigger events days even months in advance allowing them to say advertise a rally, or a planned outage, having a reliable time source does open up the power of the RC210 Repeater Controller.
For us, an accurate time allow us to beacon on the Hour, quarter past, Half past and Quarter too fully in line with the BR68. If the repeater is in use, these beacons are played at a lower deviation level thus not affecting the on going QSO.
The Hour and Half Hour beacons are Voice Identifications, the others are CW. These events are normally launched within 200mS of Zero Seconds. If we take the 1 AM Voice ID for example. At the moment the RTC rolls over to 01:00:00 the Transmitter is Keyed followed by the ID and time announcement. It is the point the transmitter is keyed is the zero second mark.
Technical overview summary:
Accuracy of approx 28nS to the RC210
Programmable number of updates per 24hr period. Set to 48 per day
Automatic building of the Almanac at start up
System Capable of processing full NMEI Sentences
Production version receives $GPRMC Packets every 1 second
Default time and Date (00:00:01 01/01/2020) sent to the RC210 at start up until GPS Signal is Acquired
GPS Acquisition normally within 2 Minutes (MAX)
STATUS Messages and Real time data displayed on LCD Screen