Introduction This article includes information on improving the sensitivity and noise temperature of receiving systems, thus improving the quality of weak VHF and UHF RF signals. There are various ways of achieving this depending on the particular circumstances. Emphasis is placed on reducing the internal noise floor, and increasing the signal-to-noise ratio. Hence, varicap or varactor tuning is very useful for rapid tuning. Tunable RF preamplifiers are useful for reducing or eliminating cross-modulation and intermodulation distortion IMD problems. Also, by only amplifying a relatively small RF bandwidth, the dynamic range is improved.
|Published (Last):||17 December 2013|
|PDF File Size:||13.3 Mb|
|ePub File Size:||10.33 Mb|
|Price:||Free* [*Free Regsitration Required]|
Introduction This article includes information on improving the sensitivity and noise temperature of receiving systems, thus improving the quality of weak VHF and UHF RF signals. There are various ways of achieving this depending on the particular circumstances. Emphasis is placed on reducing the internal noise floor, and increasing the signal-to-noise ratio. Hence, varicap or varactor tuning is very useful for rapid tuning. Tunable RF preamplifiers are useful for reducing or eliminating cross-modulation and intermodulation distortion IMD problems.
Also, by only amplifying a relatively small RF bandwidth, the dynamic range is improved. Why use a preamplifier? The second reason is because of long cable lengths, which can result in heavy signal attenuation if a preamplifier is not used at the masthead. If you live in a high noise area, where the external man-made and atmospheric noise floor is greater than 4db at 50 MHz, preamplifiers will offer little or no improvement to the signal to noise ratio. For example, man-made electrical interference from neighbors is often high enough to negate the benefits of any low-noise RF preamplifier.
Also, on certain days, power line noise can be as high as S5. When external noise is this high, indoor or masthead preamplifiers will not improve weak signals.
Apart from re-locating to a rural area, little can be done to combat this problem. Highly directional antenna systems, coupled with appropriate polarization, can often reduce man-made QRM.
Receiver noise blankers and phase cancellation can also help, but these techniques are beyond the scope of this article. At MHz frequencies, man-made noise sources will not be lower than 2dB in city or suburban areas.
Masthead TV preamplifiers with noise figures lower than 2dB are not really beneficial at VHF, because the antenna receives a constant background noise of 2dB or more.
Only in quiet rural areas will background noise be below 2dB at VHF frequencies. If narrow IF receiver bandwidths are used, for example 2. For example, MHz weak signal hams often use a masthead preamp, which have a noise figure of typically 0. RF Preamplifiers are not always needed Just because a pre-amp can increase the S meter on your receiver, there is more involved. If after installing a pre-amp, you may find spurious signals appear all over the dial!
In this case the pre-amp overloads the RF and mixer stages. The probable cause s are excessive pre-amp RF gain, bandwidth, or inferior bipolar design, etc. A poorly designed pre-amp can cause receiver problems, even though it might not degrade the basic performance of the receiver by causing overloading.
The pre-amp could have high noise, and this could make weak signal reception worse than without the amplifier. Also, the pre-amp might be unstable self-oscillating , which can cause all manner of unmodulated carriers to appear.
Bob Cooper recently commented that a receiver of that era required around 1, microvolts to produce a grainy image on the small screen, and RCA was recommending 5, microvolts. A modern TV set with 50 microvolts will produce a far better image than the version with 1, microvolts.
Typical TV tuner noise figures were around dB. In most cases, I have found that a 2dB Mosfet tunable MHz pre-amplifier offers little or no improvement on weak video signals. If noise on the TV screen shows an obvious increase, the tuner noise level is low enough, and hence a preamplifier will produce little or no improvement on weak signals.
If however no increase in noise is observed, a preamplifier will be essential. At frequencies above 88 MHz, external man-made and atmospheric noise is lower. The ET was a revolution in terms of strong signal handling and low noise RF performance. Since aligning all the RF and IF coils, the sensitivity is now optimum. Now that the tuner is quite sensitive, a 2dB noise figure BF pre-amplifier will only offer a small improvement on weak signals.
On the other hand, if a tuner is relatively insensitive or needs aligning, a 2dB noise figure preamplifier can offer a significant improvement on weak signals. During times of high external man-made or atmospheric noise, I find that the use of a low noise preamplifier, usually offers no improvement on the MHz FM band. For this reason, a sensitive FM tuner, which has optimum alignment, usually will not need additional external RF pre-amplification.
The T II features a digital signal strength meter. By tuning to a blank channel, and connecting an external FM aerial, the signal strength meter indicates a higher dB reading. Under normal low noise conditions, the increase is approximately 2dB. If the meter increases by more than dB, external noise levels are high, hence a preamplifier will produce no improvement. This means that the total RF gain is reduced, hence dynamic range is improved.
Considering the RF input is untuned, the overload and image rejection specifications are very good. I use the UA indoors, for permanent connection to my Icom R scanner. Low to medium RF gain is ideal for minimising preamp overload. This is partly why the UA has unusually good overload immunity. Cross-modulation and overload Mosfet TV and FM tuners offer superior performance in terms of strong-signal handling and freedom from cross-modulation.
Reference to the circuit diagram will reveal what types of transistors are used in the RF stage. Another advantage of Mosfets is their superior low noise performance. The gain of any RF pre-amplifier should be usually no more than 20dB. Between dB gain is usually the best compromise in terms of strong signal handling.
Another important key to minimizing overload is to only amplify a small bandwidth. Overload problems are thus greatly reduced. Total receive system noise figure High gain antennas, low loss cable, and good receivers are the most important equipment for DX reception. However, because external noise levels are lower above MHz, RF pre-amplifiers can offer considerable improvement to weak signals. Generally speaking, mast mounted pre-amps are better for DX work above MHz. Low loss coax cable is essential if the pre-amp is used indoors because every dB of coax cable feedline loss in front of the preamplifier will add that number of dB to the noise figure of the receive system.
If our receiver has a noise figure of 7dB, we add 4dB, which gives us a total of 11dB. This figure 11dB represents our total receiving system noise figure!
In contrast, if we use a 2dB noise figure preamp at the antenna, which is preceded by a 1dB loss balun, our total system noise figure is only 3dB! Quite an improvement compared to 11dB! Remember that a 6dB improvement is equivalent to making the original antenna system four times as large or going from one to four yagis! This underscores why masthead mounting of UHF preamps is important. By using a pre-amp indoors, only the noise figure of the receiver is improved. For example, a typical VHF receiver or scanner has a noise figure of approximately dB, depending on the RF front-end.
By using a 2dB pre-amp, the effective noise figure of the receiver is lowered to approximately 2dB. With a mast-mounted pre-amp, the first active transistor in the pre-amplifier sets the system noise figure, after losses associated with the input balun.
The BF pre-amp can be used at the masthead. You will need to run a shielded wire containing the DC tuning voltage for the varicap diodes, up to the masthead. The benefits of mounting the pre-amp at the masthead will be more noticeable above MHz. Bipolar transistor pre-amplifiers are usually unsuitable for serious DX reception, because of overload and cross-modulation problems. As a general rule, GaAsfet or Mosfet pre-amplifiers are the best option for weak signal reception.
This is because the terrestrial noise floor is usually no lower than 2dB from MHz. For this reason, Mosfet amps are more practical. Also, GaAsfet amps are very prone to static charges, which can destroy the device. Mosfets, on the other hand, feature protection diodes on the input, hence the device is fairly resistant to static charges.
I have built three BF pre-amps. The circuit is based around the Philips BF Mosfet transistor, which achieves a noise figure as low as 0. My version features a more modest 1. The Philips data sheets give typical noise figures for the BF as 0. Curves are provided for determining the source admittance necessary to obtain these optimum noise figures.
This type of performance can only be obtained with a dedicated pre-amp tuned to a single frequency, for example, MHz. D1 and D2 are BB varicap diodes. For band 1 operation, L1 and L2 set the basic frequency, the bandwidth being adjustable from 2 MHz to over 6 MHz by means of the two 25K linear potentiometers, which adjust the bias applied to D1 and D2 a bandwidth of less than 2 MHz might lead to instability.
This is especially important on the MHz FM version. Coils L1 and L2 are wound close spaced wire diameter using 0. Inside diameter is 0. L2 is tapped a third of the way down from the LT supply end to provide a reasonable 75 ohm match at the output. The resistance may be anywhere between approximately ohms.
The D drain voltage should be approximately 10v, while the gate 2 voltage should be around 4v. C1 is a pF trimmer: adjust for maximum output and minimum noise figure. I found the optimum setting was around 2pF trimmer plates nearly unmeshed. It is very important that C1 is soldered directly on to the input connection pin. The lead length between C1 and G1 should be short as practicable. L1 wire should be at least 0. I used 0.
BF981 Philips Semiconductors, BF981 Datasheet
I will try and buy some more stock but in the mean time. Now to make bf datasheet look pretty!!! I can send these little parts to you in ESD safe padded envelope. This bare wire spool is a datasheet thick. As a work around solution; I flipped the leads upside down with a pair bf datasheet pliers and clipped them and formed the leads and mounted them upside down on the bottom of the circuit board. They could not print the entire part numbers on these devices plus the datecodewhich I assume is forin the 5th week and make it readable so they only put the end bf datasheet the number on.
BF981 DATASHEET PDF
All the products will packing in anti-staticbag. Ship with ESD antistatic protection. We will inspect all the goods before shipment,ensure all the products at good condition and ensure the parts are new originalmatch datasheet. After all the goods are ensure no problems afterpacking, we will packing safely and send by global express. It exhibitsexcellent puncture and tear resistance along with good seal integrity. Guarantees 1.