DC-receiver 0.1-100 MHz

Fig. 1. DC-receiver seen from the front.

Building your own equipment is not difficult if you buy ready-made modules and connect them together. I wanted to to build a DC (Direct Conversion) receiver with a broad frequency range. 

Fig. 2. Direct Conversion concept.

The concept of Direct Conversion is shown in figure 2. Four modules make up a SSB/CW receiver, and all modules can be obtained ready-made!

HF-filters are usually sold as kits or ready-made. I decided to make my own filter using a piece of Veroboard. The filter's circuit diagram and the Veroboard are shown below. 

Fig. 3. The 7 MHz bandpass filter.

Fig. 4. The filter is build with leaded components on a piece of Veroboard.

The mixer is a ready-made board centered around AD831. AD831 is an active, double-balanced mixer from Analog Devices and it runs on 10 V DC at 100 mA. The required LO level is just -10 dBm and max. input on the RF-port is +10 dBm.

Fig. 5. Active mixer 0.1 - 500 MHz.

The AF-amplifier is the well-known LM386 having 46 dB amplification. I tried to find a modern substitute, but that was difficult. Many audio ICs amplify something like 26 dB, and that is too low for DC-receivers which require 40 dB amplification or more.

Fig. 6. AF-amplifier with LM386.

The VFO is the ARDU-5351 kit sold by qrphamradiokits.com. The kit includes an OLED display, a rotary encoder, a frequency generator module (Si5351A), and the Arduino Nano. I soldered all parts onto the motherboard except the Nano, which is fitted using sockets. There was no soldering of SMD-components.

Fig. 7. The VFO kit.

As the VFO output is 7 dBm, I've added a 20 dB attenuator to lower the output and comply with the LO port level of the active mixer.

Components for power distribution and the S-meter rectifier are fitted on a piece of Veroboard as shown in figure 8 below.

Fig. 8. The 10 V power supply and the S-meter rectifier.

Fig. 9. Circuit diagram.

All modules are fitted into a metal enclosure which I acquired from Conrad Electronics (item 522953). The enclosure's front is seen in figure 1 above, and the rear is seen in figure 10 below. Figure 11 shows the open enclosure.

Fig. 10. Rear side of the DC-receiver.

Fig. 11. The DC-receiver with lid removed.

Vy 73 from OZ1BXM Lars

Homepage: http://oz1bxm.dk/ 

One-valve transmitter for 7 MHz

 

Fig. 1. Valve transmitter.

Building this transmitter was inspired by an article in "Popular Electronics" 2/1955. The circuit diagram is simple, and the valve (6AQ5) can still be purchased. So I decided to build the project.

Fig. 2. Circuit diagram.

I find the circuit diagram clever. C6 and L3 are mounted on top of the chassis, and may be touched by the operator. However, the B+ voltage is neither available at the variable capacitor C6 nor at the coil L3. This is because C5 isolates the two components from B+, and they are both grounded. This precaution increases electrical safety.

High voltage is present at the bottom of the chassis. I decided to cover all components with high-voltage by plexiglass to avoid danger of electrical shock. 

Fig. 3. Top view.

Fig. 4. Bottom view. Note the plexiglass walls. 

The power supply is unregulated. When current is drawn during transmit, the B+ goes down from 215 V to 185 V, and this reduces the transmitter output to 2 W.

Fig. 5. Power supply for the transmitter.

I wish you a happy New Year, and hope for better times next year without corona-virus!

73 OZ1BXM Lars

Homepage: oz1bxm.dk

 

Replacing Network Time with BktTimeSync

I've used Network Time about 6 months. However, the PC clock deviation could be 500 ms or more during a day. This amount of drift is not acceptable - digital modes like JT65 and FT8 require less than 100 ms deviation in order to run smoothly.

A blog post by N1RWY directed me to BktTimeSync by IZ2BKT Capelli Mauro.

Main page and software download: BktTimeSync 
Alternative page for download: https://bkttimesync.software.informer.com/

Fig. 1. BktTimeSync configuration. 

My configuration is shown in figure 1. Note that connecting to an NTP-server works only if the PC firewall allows traffic on port 123. "GPS Configuration" is not filled in as I don't use a GPS device as time source.

BktTimeSync should run automatically when the PC starts up. How to add an app to run automatically at startup in Windows 10 is described by Microsoft support. 

Fig. 2. Message from time.is: You have the exact time!

Checking your PC clock can be done by visiting time.is. You'll discover if your PC clock is off. Figure 2 shows, that my PC clock has the exact time, and the deviation is just 6 ms.

I hope BktTimeSync will continue its excellent timekeeping on my PC!

Note december 2020: BktTimeSync is still running on my Win 10 PC. System time is updated every 20 min via europe.pool.nt.org. I am satisfied with it's performance.

73 OZ1BXM Lars

Homepage: oz1bxm.dk

New 23 cm transverter from SG-LAB

My new 23 cm transverter comprises a transverter module and a PA module. Both modules came fully assembled and tested from SG-LAB in Bulgaria. The transverter version is 2.3. It has an optional input port for a GSPDO (10 MHz).

Fig. 1. 23 cm transverter block diagram.

The transverter's IF is 144 MHz. RF output is 1296-1298 MHz at 2 W which is raised to 25 W using the PA. Both units have two multi-color LEDs on the front:

• Input power LED
• Output SWR LED

When a LED is green, all is well. Yellow means warning, and red means a dangerous condition. Figure 2 below shows "all well" on the PA module to the right. The transverter LEDs display yellow and green. The reason for yellow is low input power. This is necessary not to overload the PA module. 

Fig. 2. LED indications on transverter and PA during transmit.

The IF-transceiver is Yaesu FT-847. It has a STBY port on the rear panel which goes low during TX. The STBY port is connected to the transverter's PTT port via a coax-cable.  

Fig. 3. Transverter and PA inside the alu-box. 

Figure 3 shows the transverter and the PA mounted inside a Hammond 1550J alu-box. The RF ports on both modules are fitted with SMA-female connectors. The modules are interconnected with short pieces of RG316D cable having SMA-male connectors. 

The PA  module becomes warm, but not hot during transmit. The PA module contains a pre-amp with 10 dB gain and NF 0.8 dB.

Fig. 4. The transverter is mounted below the 23 cm antenna.

The water-proof alu-box containing the transverter is mounted below the 23 cm antenna (fig. 4). The middle antenna is a 10-element yagi for 70 cm, and the lower antenna is a 6-element yagi for the 2 meter band.

Receiving 23 cm beacons portable

Last Saturday, I collected my 14 element yagi for 23 cm, my MKU13G2B Kuhne transverter, and my 144 MHz IF radio (Kenwood TR-751E). A rechargable 12 V battery, a camping table and a folding chair was also included.

I went to Trehoje JO46HE which is 102 m ASL. I wanted to listen for Danish 23 cm beacons. 

My 23 cm portable rig at Trehoje JO46HE.

The nearest beacon is OZ5SHF in JO45VX, 76 km away. The signal was stronger than I had anticipated: S9. The beacon is at 205 m ASL, and that certainly helps propagating a strong signal! You can hear my audio recording of OZ5SHF:

OZ5SHF audio

Beacon OZ7IGY in JO55WM is 216 km away. The signal was weak, but I could hear the PI4-tones. If I had brought my laptop, I would have been able to detect the PI4-signal using the sound card and the  PI-RX program.   

The beacon OZ1UHF 162 km away was very weak. I could hear it, but not read the morse code.

Next step is to become QRV on 23 cm transmit from my home QTH.

73 from OZ1BXM oz1bxm.dk

Noisy PC Speakers - and a Solution

My PC-speakers are pretty standard: A pair of Creative A50 powered with 5 V DC from an USB-port.

I noticed noise coming from the speakers, when no audio signal was present. Changing USB-port did not help. I seemed like my USB ports were noise polluted! When I connected a power-bank (lithium-ion battery) to the speakers, the noise disappeared, but during pauses in the audio, the power-bank shut itself down!

I decided to build a noisefree 5 V DC PSU using a transformer with iron core and a 7805 which has 68 dB ripple rejection and low output noise. The 7805 is thermal overload protected and short-circuit protected. I had all parts laying around, so the building cost was nil.

Figur 1. The noisefree PSU.

Figure 2. Noisefree PSU with open lid.

Figure 3. AC Adaptor. 

Figure 3 shows the transformer which is an old AC adapter with iron core. It is heavier than modern AC adapters! The advantage: no switching technology is involved!

Figure 4. Circuit diagram.

The rectifier bridge is an integrated piece, but four discrete 1 A diodes can be used as well. I found C1, C2, and 7805 in the drawer. The circuit board has holes and solder islands, but no tracks. Figure 5 shows how the component wires are connected on the bottom side. The alu-cabinet is Hammond 1590A.

Figure 5. The circuit board seen from the bottom.

The PC speaker's power consumption is low. The noisefree PSU does not get warm at all.

Now I can enjoy music and speech without background noise - and a good feeling of having improved something!

73 from OZ1BXM
My homepage: http://oz1bxm.dk


 

Replacing Meinberg NTP with Network Time

I purchased a new PC last month with Windows 10 preinstalled. The Windows specs are shown in figure 1.

Figure 1. My new Windows 10.

I had problems after I installed Meinberg NTP. Whenever I booted the PC, the system clock was set by Meinberg, but then time discipline was lost. After some hours, the system clock could deviate as much as 1 sec. It seemed like Meinberg could not control the system time. I worked with this problem for a while, and then I decided to uninstall Meinberg NTP and try another piece of software for time control.

I need accurate time keeping because I'm using weak signal digital protocols in amateur radio. A time deviation of less than 100 ms is required with JT65. I use JT65 when I'm bouncing radio signals off the Moon and back to Earth (distance 800,000 km).

Figure 2. Network Time.

Network Time was easy to install, and it worked right away. Under "Settings" I set "Update Interval" to 20 minutes because I wanted frequent updates. Remaining options were left at the factory settings.

Before installing Network Time I opened UDP port 123 (inbound/outbound) in Windows Defender Firewall. This port is used by the NTP (Network Time Protocol). I named the rule "UDP-port-123-time1", see figure 3.

Figure 3 Windows Defender Firewall (Windows 10).

I changed "Set the time automatically" from On to Off as shown in figure 4. This tells Windows not to use W32time.

Figure 4. Date and Time (Windows 10).

You can check the accuracy of your computer clock by visiting this page.

Vy 73 from OZ1BXM Lars
Webpage: oz1bxm.dk